How about a e-commerce site in the Fediverse where you can either sell second hand stuff (like ebay) or sell stuff you made or paid people make (like amazon or etsy) but with multiple instances with different moderation, shipment costs and some might require you to have id obviously their will be sketchy instances or users but the community could make a list of instances/users to avoid or to be careful of at least, and most the users on this list would be user who their instance hasn’t banned (which might put the instance into disrepute, since if they are notorious enough to land on this list you should probably ban them) and the people who are in charge of this list can get reports from user and they will have to conform from other buyers who bought from that individual to make sure people aren’t lying because they don’t like the person or something, it could be federated/integrated with services like mastodon, peertube and possibly lemmy where you can sign in with it with your account from these services, but you have to make an account with this service to actually sell anything, but user from these other platforms could link their shop on this service on their mastodon post and/or account and peertube videos/account (sorry if I got carried away), what do you guys think?
In Belgium we have 2dehands. Not perfect or open source but miles better than Facebook marketplace.
No payments go through the platform. People meet in person and use cash or use a bank transfer or 3rd party payment like payconiq to buy and sell things. It works perfectly.
Not everything has to have an integrated shopify style payment platform. Most of the population is perfectly capable of exchanging money.