One button checks willhaben for newly listed devices with a broken screen. It works out what each one is worth after a screen swap and shows only the ones that can make your target profit.
It runs your searches for broken iPhones, Samsung Galaxy S phones and MacBook Airs from private sellers, newest first. Then it drops anything iCloud-locked, water-damaged or sold for parts, and compares each asking price with the most you can pay.
Listings you haven't seen before are marked NEW. Nothing is stored except your settings and which listings this browser has seen.
The listing mentions screen damage, no warning signs, and the asking price is at or under the most you can pay for your profit target.
Up to 25% over your limit, so worth an offer. Or under your limit but the listing doesn't say what's broken, or mentions something to ask about first.
Meet in person. The seller removes the phone from their Apple ID or Google account in front of you, and it restarts to the setup screen. Check Face ID and that it charges.