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Build a Price Comparison Tool in 15 Minutes with the Marketplace Price API

#tutorial

Ever tried to find the best price for something across multiple marketplaces? You'd need to open OfferUp, Poshmark, Reverb, TCGPlayer — each with different search interfaces and no way to compare side by side.

The Marketplace Price API solves this with a single unified /search endpoint that queries multiple marketplaces simultaneously and returns normalized results.

In this tutorial, we'll build a CLI price comparison tool in under 15 minutes.

What We're Building

A Node.js script that:

  1. Takes a search query as input
  2. Searches across 4 marketplaces in parallel
  3. Shows a sorted comparison table with the best prices

Prerequisites

Step 1: Get Your API Key

Subscribe to the Marketplace Price Tracker API on RapidAPI. The free tier gives you 500 requests/month — plenty for this tutorial.

Copy your X-RapidAPI-Key from the API dashboard.

Step 2: Set Up the Project 

mkdir price-compare && cd price-compare
npm init -y
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Create a .env file:

RAPIDAPI_KEY=your_key_here
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Step 3: Build the Comparison Tool

Create compare.mjs:

const API_KEY = process.env.RAPIDAPI_KEY;
const BASE = "https://marketplace-price-tracker.p.rapidapi.com";
const HEADERS = {
  "X-RapidAPI-Key": API_KEY,
  "X-RapidAPI-Host": "marketplace-price-tracker.p.rapidapi.com",
};

const query = process.argv[2];
if (!query) {
  console.error("Usage: node compare.mjs <search query>");
  process.exit(1);
}

console.log(`\nSearching for "${query}" across 4 marketplaces...\n`);

// Search all marketplaces in parallel
const endpoints = [
  { name: "Reverb", path: `/reverb/search?query=${encodeURIComponent(query)}&limit=5` },
  { name: "TCGPlayer", path: `/tcg/search?query=${encodeURIComponent(query)}&limit=5` },
  { name: "OfferUp", path: `/offerup/search?query=${encodeURIComponent(query)}&limit=5` },
  { name: "Poshmark", path: `/poshmark/search?query=${encodeURIComponent(query)}&limit=5` },
];

const results = await Promise.allSettled(
  endpoints.map(async ({ name, path }) => {
    const res = await fetch(`${BASE}${path}`, { headers: HEADERS });
    if (!res.ok) return { name, items: [] };
    const data = await res.json();
    const items = (data.results || data.listings || []).map((item) => ({
      marketplace: name,
      title: item.title || item.productName || item.name,
      price: parseFloat(item.price || item.marketPrice || item.lowestPrice || 0),
      condition: item.condition || "N/A",
      url: item.url || "#",
    }));
    return { name, items };
  })
);

// Flatten and sort by price
const allItems = results
  .filter((r) => r.status === "fulfilled")
  .flatMap((r) => r.value.items)
  .filter((item) => item.price > 0)
  .sort((a, b) => a.price - b.price);

if (allItems.length === 0) {
  console.log("No results found. Try a different search term.");
  process.exit(0);
}

// Display results
console.log("Price".padEnd(10) + "Marketplace".padEnd(14) + "Condition".padEnd(12) + "Title");
console.log("-".repeat(80));

for (const item of allItems.slice(0, 20)) {
  const price = `$${item.price.toFixed(2)}`.padEnd(10);
  const mp = item.marketplace.padEnd(14);
  const cond = (item.condition || "N/A").slice(0, 10).padEnd(12);
  const title = item.title?.slice(0, 44) || "—";
  console.log(`${price}${mp}${cond}${title}`);
}

console.log(`\nFound ${allItems.length} results across ${results.filter((r) => r.status === "fulfilled").length} marketplaces`);
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Step 4: Run It 

RAPIDAPI_KEY=your_key_here node compare.mjs "fender stratocaster"
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Output:

Searching for "fender stratocaster" across 4 marketplaces...

Price     Marketplace   Condition   Title
--------------------------------------------------------------------------------
$149.99   OfferUp       Good        Fender Squier Stratocaster Electric Gui
$225.00   Poshmark      N/A         Fender Player Stratocaster HSS Buttercr
$349.00   Reverb        Excellent   Fender Player Stratocaster 2019 3-Color
$425.00   Reverb        Very Good   Fender American Stratocaster Sunburst
$599.99   Reverb        Excellent   Fender American Pro II Stratocaster
...

Found 18 results across 4 marketplaces
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Available Endpoints

Endpoint Description
GET /reverb/search Reverb music gear listings
GET /reverb/price-history Historical price data for Reverb items
GET /tcg/search TCGPlayer card listings
GET /tcg/price TCGPlayer card price lookup
GET /offerup/search OfferUp local marketplace listings
GET /poshmark/search Poshmark fashion resale listings

Use Cases

  • Resellers: Find arbitrage opportunities across marketplaces
  • Collectors: Track TCG card prices, find the best deals on vintage gear
  • Shoppers: Compare prices before buying
  • Discord Bots: Power price-check commands with real-time data
  • Price Tracking Apps: Build dashboards with historical pricing

What's Next?

  • Add a simple web UI with React or Svelte
  • Store historical prices in SQLite for trend analysis
  • Set up a cron job to check prices daily
  • Build a Discord bot with /price-check commands

The Marketplace Price Tracker API free tier includes 500 requests/month — enough to build and test any of these.

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