I just realized the Swedish slang for flea market would work a charm. Loppis. It even sounds like a hip app. Someone quick make it an open fedi platform before the capitalists take it!
Edit: It's short for loppmarknad. Loppa = flea. Marknad = market.
I don't know how, but i kinda imagined it even though i don't speak a word of German (nor i am an English native speaker). It just... uh, sounds like it.