DEV Community: MOON

I shipped a paid SaaS in 3 days with no user accounts: the billing setup and 3 bugs that bit me

MOON — Thu, 17 Sep 2026 09:17:45 +0000

I write a small blog on the side. Every time I picked a topic I hit the same wall: Ahrefs and Semrush are $130+ a month, and even when I had the numbers, "volume 590, difficulty 17" didn't tell me whether to write the post or skip it.

So I built Keyword Brief. You type a keyword and get a plain-language GO / MAYBE / SKIP, real Google volume and difficulty, a read of what the top 10 pages actually cover, and an outline with the gaps they left open. It deliberately does not write the article for you.

This post is about how it's built, because the part I'm happiest with is the part users never see: there are no accounts. No signup, no password reset, no user table, and it still takes payments.

The stack

Next.js on Vercel
DataForSEO for search volume, difficulty, and the live top 10
A plain fetch + cheerio pass to read the H2/H3 headings off each ranking page (free, and more honest than a "content score")
Claude to turn those headings and the People Also Ask questions into an outline
Lemon Squeezy for subscriptions and license keys

One check costs me roughly $0.10. Pro is $19.90 a month.

Billing without accounts

Here's the whole model:

Free tier: 3 checks per browser, counted in an HMAC-signed cookie. You can't edit the number, and I don't need a database row to remember you.
Paid: a Lemon Squeezy subscription with "Generate license keys" turned on.
One-click activation: Lemon Squeezy lets you put variables in the confirmation button link. Mine is:

https://keyword-brief.vercel.app/activate?key=[license_key]

After checkout the customer clicks one button, lands on /activate, and the page activates the key for that browser and strips it from the URL. No copy and paste. The same link is on the receipt email as a fallback.

Staying honest: the server re-validates the stored key at most once an hour. Cancel or refund, and Pro switches off on its own.

// First time a key is seen in this browser: register the browser as one "instance" of the key.
const json = await lsPost("activate", {
  license_key: key,
  instance_name: `web-${Date.now().toString(36)}`,
});
if (!json.activated) return { ok: false, reason: json.error };
// cookie = sign(`${key}|${json.instance.id}`)

The expensive call (the Claude outline) is a second request, so it's gated too: the first request issues a short-lived, keyword-bound token, and the outline endpoint refuses to run without it. Otherwise anyone could skip the free counter by calling that endpoint directly.

Yes, someone can clear cookies to get 3 more free checks. For a $19.90 tool that's a trade I'll take in exchange for zero signup friction.

Bug 1: every new customer was "inactive"

My first test purchase failed with "This key is inactive."

A fresh Lemon Squeezy key has status inactive. That doesn't mean invalid. It means "not activated on any device yet" (0/5). I was calling /validate and rejecting anything that wasn't active, which means I was rejecting every brand-new paying customer.

The fix is to call /activate the first time you see a key (that's what flips it to active and enforces the device limit), then /validate with the instance_id afterwards, and /deactivate when the user removes the key so the slot is freed.

Bug 2: structured outputs ignored my array limits

I defined the outline schema with zod, including things like "at most 4 points per section":

points: z.array(z.string()).min(1).max(4)

About one request in three blew up with a validation error. Structured outputs guarantee the shape, but array length constraints aren't enforced during generation. The model wrote 5 good points, and my own validator threw the whole response away.

Now the schema has no length limits. The lengths live in the prompt, and I trim in code:

sections: o.sections.slice(0, 9).map((s) => ({ ...s, points: s.points.slice(0, 5) }))

Bug 3: "top 10" returned 6 results

DataForSEO's SERP depth: 10 doesn't mean 10 organic results. It counts every element on the page: videos, People Also Ask, image packs. I was getting 6 organic links and confidently telling users "the top 10 has plenty of small sites."

Ask for depth: 30, filter to type === "organic", take the first 10.

A feature I didn't plan

I'm not a native English speaker, and while testing I typed "how to fishing well". No data at all, because nobody searches that.

Returning "no demand" would have been wrong and a bit insulting. Now, when a phrase has no data, Claude rewrites it into 4 to 6 natural queries, I verify those against DataForSEO in one batch call, and the page shows what people actually search, with volumes: fishing tips (1,600), fishing tips for beginners (320), how to catch more fish (140). Those "no data" lookups don't count against the free checks.

Perceived speed

A full check took about 36 seconds, nearly all of it the outline. I split it in two: the verdict, numbers, and page-one analysis render in about 7 seconds, and the brief fills in underneath while you're reading. Same total time, completely different feel.

Try it

keyword-brief.vercel.app has 3 free checks with no signup. Pro checkout opens this week.

The thing I most want to hear: try a keyword you know well and tell me if the verdict is wrong. The GO / MAYBE / SKIP rules are simple and transparent on purpose, and real counterexamples are how they get better.

I pulled the real sales numbers from Daiso Korea's website. Here are the 48 products Koreans actually buy.

MOON — Wed, 09 Sep 2026 09:01:57 +0000

Every "Korean Daiso haul" video tells you what one person bought. I wanted to know what everyone buys.

It turns out Daiso Korea's online mall (daisomall.co.kr) exposes a number most retailers hide: the cumulative units ordered for every product. So I collected 2,349 unique products across 15 categories — beauty, skincare, kitchen, cleaning, storage, stationery, pets, snacks — and ranked them by real sales volume. Together these 2,349 products have sold 24.8 million units.

Here is what the data says. (Prices in Korean won; ₩1,000 ≈ US$0.74.)

The headline numbers

33 of the 48 best-sellers are beauty or personal care. Daiso Korea is, by volume, a beauty store that also sells kitchen towels.
23 of the 48 cost ₩1,000 (~$0.74) or less. Nothing costs more than ₩5,000 ($3.70) — 13 sit right at that ceiling.
Average rating of the 48: 4.80 / 5. Not one of them is sold out — Daiso keeps its winners in stock.
The single best-selling item is a pack of wet wipes: 994,677 units. Not glamorous, but that's a million packs of one SKU from one online channel.

Where the volume is (by category, whole sample)

Category	Products sampled	Units sold
Beauty & hygiene	906	13.6M
Cleaning & bathroom	325	4.6M
Storage & organization	175	1.6M
Kitchen	129	1.5M
Food & snacks	206	1.0M
Stationery	187	0.9M
Sports & hobby	200	0.8M
Pet supplies	178	0.6M

Beauty alone is 55% of all units in the sample.

The 50 best-selling products at Daiso Korea (by cumulative units)

Daiso lists many products twice (single unit and bulk box) sharing the same sales counter; duplicates were merged, so 50 listings = 48 unique products.

#	Product	Price	Units sold	Rating (reviews)
1	Eclat clean wet wipes, cap-type, 150 sheets	₩1,000	994,677	4.8 (25,764)
2	Living pull-out kitchen towels, 150 sheets	₩1,000	770,852	4.9 (20,361)
3	Tempo toilet paper roll, 24 m	₩500	590,389	4.9 (4,923)
4	VT PDRN Glow sheet mask (1 sheet)	₩1,000	499,272	4.9 (7,781)
5	Mommy Care sea grape skin pads, 80 pads	₩5,000	342,377	4.9 (18,304)
6	Limescale & urine-stone remover, 500 ml	₩3,000	288,291	4.7 (15,913)
7	Egg capsule wash-off pack	₩500	276,379	4.8 (6,926)
8	Boncep Vita C freeze-dried double-shot ampoule kit	₩5,000	267,685	4.8 (14,596)
9	Celderma Daily azulene calming sheet mask	₩2,000	257,712	4.8 (6,562)
10	Puppy moist cushion puff	₩1,000	228,709	4.8 (6,796)
11	"Personal info eraser" (ink to hide your address on parcel labels), lavender	₩1,000	218,965	4.8 (12,019)
12	VT PDRN Glow toner, 200 ml	₩5,000	212,668	4.8 (13,854)
13	Baking-soda cleaning wipes, 100 sheets	₩1,000	204,761	4.8 (10,611)
14	Madeca 21 Teca Solution soothing mist toner, 200 ml	₩5,000	201,463	4.9 (15,615)
15	Air cushion puff, 2-pack	₩1,000	199,857	4.8 (9,436)
16	Puppy glow cushion water puff	₩1,000	174,653	4.8 (5,372)
17	2-in-1 fruit fly trap set	₩2,000	166,858	4.8 (9,520)
18	Pack Me In hydrogel V-line mask	₩1,000	157,695	4.7 (5,190)
19	Bathroom cleaning sponges, 10-pack	₩2,000	153,081	4.8 (7,839)
20	Pull-out kitchen towels, 150 sheets (2nd listing)	₩1,000	150,732	4.9 (4,520)
21	Rectangular storage box, 26×10×9 cm	₩1,000	150,133	4.9 (4,813)
22	VT Reedle Shot 100 first ampoule, 2 ml × 6	₩3,000	149,840	4.8 (16,760)
23	VT Reedle Shot 300 first ampoule, 2 ml × 6	₩3,000	142,578	4.9 (16,154)
24	Foam-type "magic cleaner", 550 ml	₩3,000	140,933	4.7 (5,442)
25	Fine-mist cosmetic spray bottle, 120 ml	₩1,000	137,129	4.8 (9,696)
26	Boncep Retinol 2500 IU wrinkle-shot perfector, 15 ml	₩5,000	136,202	4.8 (10,462)
27	Dong-A laundry stain stick	₩1,500	120,619	4.7 (8,355)
28	The Lab by Blanc Doux hyaluronic glow booster ampoule, 30 ml	₩5,000	117,256	4.8 (7,287)
29	Homestar 3X foam bleach cleaner (floral)	₩3,000	107,982	4.9 (5,629)
30	Puppy fitting cushion puff	₩1,000	106,729	4.8 (3,193)
31	VT PDRN Glow cream, 50 ml	₩5,000	104,014	4.8 (5,514)
32	The Lab by Blanc Doux hyaluronic glow cream, 50 ml	₩5,000	101,822	4.8 (6,992)
33	Medi-Peel Melanon X dark-spot cream, 30 ml	₩5,000	100,437	4.7 (4,457)
34	Steambase daily eye warmer, cypress scent	₩1,000	98,611	4.8 (3,022)
35	Portable lens cleaner wipes, 24-pack	₩1,000	96,052	4.8 (6,259)
36	Vaseline intensive moisture sheet mask	₩1,000	95,926	4.8 (3,631)
37	Induction-cooktop grease wipes, 60 sheets	₩2,000	95,671	4.8 (5,327)
38	Madeca 21 Teca Solution soothing cream, 50 ml	₩5,000	93,664	4.8 (8,528)
39	Glass & mirror cleaning wipes, 25 sheets	₩1,000	93,106	4.6 (4,969)
40	Boncep retinol serum sheet mask	₩1,000	86,650	4.8 (2,884)
41	Boncep Retinol 500 IU serum, 30 ml	₩5,000	86,072	4.8 (7,323)
42	Celderma Daily collagen essence gel mask	₩1,000	85,652	4.7 (2,326)
43	Real Barrier Cera Base calming moisturizing cream, 50 ml	₩5,000	85,421	4.8 (6,397)
44	VT PDRN Glow lifting mask + essence	₩2,000	83,381	4.8 (2,043)
45	Mini powder puff, 2-pack	₩1,000	81,183	4.9 (5,539)
46	3-tier stacking drawer	₩3,000	79,704	4.8 (4,721)
47	Pation Noscarnine first-step scar ampoule, 30 ml	₩5,000	79,630	4.8 (4,628)
48	Potato capsule wash-off pack	₩500	78,964	4.8 (3,052)

Five things the numbers say

1. VT is the brand that owns Daiso Korea. The Reedle Shot ampoules (the ones that went viral on TikTok in 2024–25) sit at #22 and #23 with ~145k units each, but VT's PDRN "Glow" line is now bigger: the PDRN sheet mask alone has sold 499k units at ₩1,000, and the PDRN toner, cream and lifting mask are all in the top 50. Six VT products in the top 48. If you're sourcing K-beauty, VT-at-Daiso is the story of the year.

2. "Dermocosmetic actives at dollar-store prices" is the winning formula. Freeze-dried vitamin C (Boncep, #8), retinol 2500 IU (#26) and 500 IU (#41), PDRN (VT), madecassoside (Madeca 21, #14/#38), noscarnine scar ampoule (#47), a dark-spot cream (#33). Koreans are buying actives, not just sheet masks — at ₩3,000–5,000 ($2.20–3.70).

3. Sheet masks are still the volume engine. Nine sheet/capsule masks in the top 48, all ₩500–2,000. The ₩500 egg capsule pack (#7) and potato capsule pack (#48) are Daiso originals that don't exist anywhere else.

4. Makeup sponges are a quiet monster. Five cushion puffs in the top 48 (#10, #15, #16, #30, #45) — around 790k units combined. Nobody talks about them; everybody buys them.

5. Cleaning products are the sleeper category. A limescale remover with 288k units and 15,913 reviews (#6). Baking-soda wipes, induction wipes, glass wipes, a fruit-fly trap with 167k units. If you sell home goods, Korea's cleaning aisle is a product-development goldmine.

Methodology

Data was collected on 2026-09-07 from daisomall.co.kr search results for 15 category keywords (cosmetics, skincare, sheet masks, lip, sunscreen, cleansing, shampoo, storage, kitchen, cleaning, bathroom, stationery, pets, camping, snacks), 3,072 listings → 2,349 unique products. The totalSold field is Daiso's own cumulative order count per listing. This is a category sample, not the full catalogue — there may be best-sellers in categories I didn't search.

I collected it with a scraper I built and published on Apify (Daiso Korea Scraper — it returns price, rating, review count and units sold per product, and can also pull the reviews). If you want to run it yourself, I wrote a step-by-step walkthrough — no code needed. Happy to answer questions about the data or the method in the comments.

How to get Olive Young and Daiso Korea product data as JSON — no code, no Korean IP

MOON — Wed, 09 Sep 2026 08:28:58 +0000

If you sell, source, or research K-beauty, the two Korean retailers you actually want data from are Olive Young (Korea's #1 beauty retailer, ~1,300 stores) and Daiso Korea (the ₩1,000 store where VT Reedle Shot went viral). Both have Korean-only websites, both block most scrapers, and neither has a public API.

This is a 5-minute walkthrough for getting their data as clean JSON or CSV using three small scrapers I published on Apify. No coding required; there's an API/Python section at the end if you want to automate it.

What you can get

Source	What it returns
Olive Young search	Products for any keyword (Korean or English), ordered by Olive Young's popularity ranking: price, sale price, discount %, rating, review count, brand (Korean + English), category, sold-out / coupon / today's-special flags
Olive Young reviews	Reviews for any product: text, star rating, date, photos, verified repurchase flag, one-month-use flag, helpful votes, reviewer skin type / tone / concerns
Daiso Korea	Products for any keyword with price, rating, review count and cumulative units sold — plus reviews with repurchase flag, attribute ratings and photos

Step 1 — Create a free Apify account

Go to apify.com and sign up (Google login works). The free plan includes $5 of platform credit every month, which is enough for a few thousand results — no card needed to try this.

Step 2 — Open the Actor and enter a keyword

Apify calls each scraper an "Actor". Open one of these:

Click Try for free. You'll land on an input form. For the Olive Young search scraper:

Keywords: one per line. Korean gives the best results (선크림 = sunscreen, 토너 = toner, 앰플 = ampoule), but English brand names work too (anua, mediheal, cosrx).
Max items: how many products to collect in total. Start with 100.
Leave proxy settings on the default.

Click Start. A typical run takes 10–30 seconds.

Step 3 — Look at the results and export

When the run finishes, open the Output tab. You'll see a table with an image, name, prices, rating and review count per product. Click Export to download it as JSON, CSV, Excel, or XML, or copy the dataset URL to load it directly into Google Sheets.

A single Olive Young product row looks like this:

{
  "keyword": "선크림",
  "position": 1,
  "totalResults": 614,
  "goodsNo": "A000000232672",
  "name": "[미니어처 단독기획] 메디힐 마데카소사이드 수분 선세럼 흔적 리페어 50+50+15g 기획",
  "brand": "메디힐",
  "brandEn": "mediheal",
  "originalPrice": 34900,
  "salePrice": 21200,
  "discountRate": 39,
  "currency": "KRW",
  "rating": 4.9,
  "reviewCount": 16595,
  "isRepurchase": null,
  "hasCoupon": true,
  "isTodaySpecial": true,
  "isSoldOut": false,
  "categoryMid": "선케어",
  "url": "https://www.oliveyoung.co.kr/store/goods/getGoodsDetail.do?goodsNo=A000000232672"
}

Step 4 — Get the reviews for a product

Copy the goodsNo value from the search results (e.g. A000000232672) and paste it into the Products field of the Oliveyoung Review Scraper. You can paste product URLs instead of IDs.

Useful options:

Sort by: newest first (default), most helpful, highest / lowest rating.
Review type: all, photo reviews only, one-month-use reviews, or repurchase reviews only.
Max reviews per product: 100 by default; a hit product has 10,000+.

Each review comes back with rating, text, date, isRepurchase, isOneMonthUse, images, helpfulCount, and a reviewer object with skinType, skinTone and skinConcerns in English (Olive Young stores these as codes; the Actor translates them).

Step 5 — Daiso Korea: products with real sales volume

The Daiso Korea Scraper works the same way. Enter keywords (화장품 cosmetics, 마스크팩 sheet mask, 주방 kitchen, 수납 storage…) and you get every matching product with a field most retailers never expose:

{
  "name": "[수분충전]VT PDRN 광채 시트 마스크 1매 25 g",
  "price": 1000,
  "rating": 4.9,
  "reviewCount": 7781,
  "totalSold": 499272,
  "categorySmall": "시트팩",
  "isSoldOut": false
}

totalSold is Daiso's cumulative order count for the listing. Sort your export by that column and you have Daiso Korea's real best-seller list for any category.

Switch the Actor's Mode to reviews and paste productNo values to get review text, star rating, repurchase flag and Daiso's structured attribute ratings ("thickness: thicker than it looks", "durability: sturdy").

Step 6 — Schedule it (price / trend monitoring)

On any finished run, click Save as a task, then Schedule the task (daily, weekly, cron). Each run writes a fresh dataset; you can push it to Google Sheets, S3, a webhook, or Slack from the Integrations tab. That's a K-beauty price/rating monitor for a few cents a day.

Optional — Run it from code

Every Actor has a REST API. With the Apify Python client:

from apify_client import ApifyClient

client = ApifyClient("YOUR_APIFY_TOKEN")

run = client.actor("cocomoon/oliveyoung-scraper").call(run_input={
    "keywords": ["선크림", "toner"],
    "maxItems": 200,
})

for item in client.dataset(run["defaultDatasetId"]).iterate_items():
    print(item["brandEn"], item["name"], item["salePrice"], item["rating"], item["reviewCount"])

Or with curl:

curl -X POST "https://api.apify.com/v2/acts/cocomoon~oliveyoung-scraper/run-sync-get-dataset-items?token=YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"keywords": ["선크림"], "maxItems": 50}'

The Actors also work as tools for AI agents through Apify's MCP server, so Claude, Cursor or a custom agent can query Olive Young or Daiso directly.

Pricing and limits

All three are pay-per-result: $3–4 per 1,000 results plus a fraction of a cent per run, with platform usage included. The $5 monthly free credit covers roughly 1,000+ results, so you can evaluate the data before paying anything.

Common questions

Do I need a Korean IP? No. The Actors handle proxies; the review scraper uses Korean residential proxies by default because Olive Young's review API blocks datacenter IPs.

Korean or English keywords? Both work on Olive Young. Korean returns more results; English works for brand names. Daiso is Korean-only.

Is it legal? The Actors read publicly listed product and review data — no login, no personal data (reviewer names on Daiso are masked by the platform; Olive Young exposes only nicknames). You're responsible for how you use the data.

Something broke? Open an issue on the Actor page — retailers change their sites and I maintain these.

I'm a marketer in Seoul who works with Korean retail data daily; I built these because every foreign brand and seller I talked to was copying Olive Young rankings by hand. If there's a Korean platform you need data from, tell me in the comments.