My wife buys and resells stuff from car boot sales, charity shops, and flea markets. A few months ago I started building her a free iOS app to track it all — FlipperHelper. It's on about 70 downloads now, no revenue, no ads.
The biggest feature request I kept getting was some way to answer a simple question: was that sourcing trip actually worth it?
The problem
When you go to a market you spend money in a lot of different ways. Entry fees, fuel or train tickets, maybe parking. Then you buy items — sometimes 5, sometimes 20. Some sell within a week, some sit for months.
The expenses and the items live in completely different timelines. By the time the last item sells, you've forgotten what you even spent on that trip. My wife was trying to track this in a spreadsheet and it was a nightmare to maintain.
What I built
The feature is called Hauls. You create one, pick the dates of the trip, and the app shows all the items you bought in that period. You select which ones belong to this trip. Then you pick the expenses — entry fees, transport, whatever else. The app calculates total cost vs total earned.
The key thing is it updates over time. You sell an item three months later and the haul profit changes. So a trip that looks like a loss in week one might be profitable by month three. That's the whole point — you can look back at any trip and see where it actually ended up.
Decisions I had to make
How to handle shared expenses. If you drove your car to three markets that month, which trip gets the fuel cost? I thought about expense splitting — assign 40% to one haul, 60% to another. But that adds complexity for something most users would just estimate anyway. So I kept it simple: one expense belongs to one haul. You can always create a separate expense for each trip if you want to split manually.
Date-based vs manual item selection. The app pre-filters items by the haul dates, but you can add or remove any item. This handles the case where you bought something the day before a trip but it was clearly for that sourcing run. Dates as a starting point, manual override when needed.
What counts as profit. Haul profit = total sold amount from items in this haul, minus item purchase prices, minus haul expenses. Unsold items show their purchase cost as unrealised investment so you can see how much money is still tied up. This was important — if you spent 200 on a trip and sold 150 worth, but you've still got 100 in unsold stock, that's not a 50 loss. It's 50 in profit potential sitting on a shelf.
Cash vs card tracking
This one came directly from my wife. She said at the end of the month the card spending shows on the bank statement but the cash is invisible. At car boots everything is cash. She wanted to know exactly how much went out in cash vs card.
Simple addition — just a toggle on each expense — but it answered a real question she had about where the money was actually going.
Share cards
We already had shareable cards in the app for individual sold items. Extended the same system to hauls — you can share your trip results as a card on social media. Took maybe a day to build because the template system was already there.
What's next
Working on a few things but honestly the best features have come from watching my wife use the app and hearing what's annoying. The cash/card thing would never have occurred to me. The hauls feature came from a real France trip where we genuinely had no idea if we made money.
If you're building a side project, having one dedicated user who actually depends on it is worth more than 1000 hypothetical users.
FlipperHelper is free on the App Store if anyone wants to check it out. iOS only for now.
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