I'm the solo founder of CheapCarRent, an Estonia-based car rental marketplace. Most of the work is not the website, it is connecting rental companies' systems so their cars and prices show up in one search. Here are the places where those systems disagree the most. Maybe it saves someone an afternoon.

1. Locations are the hardest part. One system gives you an airport code, another a free-text branch name, another coordinates, and some give only a city. If you resolve "Vienna" to the wrong country code once, customers will see cars in the wrong city. We now verify that every offer has a pick-up point that can actually be placed on a map, and drop offers that can't.

2. "Deposit" and "insurance" get mixed up. Some APIs return the deposit as an insurance line with a price, and a naive mapper then shows the customer a "paid insurance" item that is really a refundable hold. Always check whether a money field is a charge or a hold.

3. Insurance is not one field. Some suppliers return selectable cover levels, some return a fixed text, some return nothing at all. The UI has to handle all three without looking broken.

4. Car class is a guess. Some suppliers send an ACRISS/SIPP code, some a free-text category, some only a model name. If you want an "economy" filter you have to build your own rules and say so when you publish numbers.

5. Availability is optimistic. Search results can include cars that are marked full for the dates, or whose booking endpoint refuses the request. Test a real booking flow before you show a supplier's cars as bookable.

6. Booking permissions differ. Many APIs give read-only keys first and write access later. Plan for a "coming soon" state where the cars are listed but booking stays off.

7. Terms and conditions live somewhere else. Age limits, mileage, fuel policy and cross-border rules are often only on the supplier's website. Showing a clean summary of them before payment saves a lot of support emails.

If you build travel or marketplace integrations, I'd be glad to compare notes in the comments.

More about the project: https://cheapcarent.com/