By flea market app, you mean like where people list stuff to sell? I have tried a few, but people are often flaky and don't show up to buy. eBay is a pain sometimes, but at least I can pack and ship the stuff whenever I want.
Yeah, a lot of my stuff is going to be location specific (to the USA). But hopefully someone that wants to step up their side-hustle game will find something similar in their neck of the woods.