When I ran r/snackexchange on reddit, we never had someone poison the snacks but 5%~ of the trades failed/were scams and we had several scary doxxing incidents. Shipping costs, especially internationally, are much higher than people are prepared for while customs agencies are really inconsistent between countries.

Exchange communities are a neat ecosystem but you either need strict top-down moderation (exposing personal information to the mods) or strict infosec practices from the users and a high degree of trust. Once I realised that I didn't want to do the former and the latter couldn't exist on reddit, I was really uneasy with the risk it exposed users to and my complete inability to protect them. That culminated in being on the phone with the Viennese police early in the morning while a user threatened to commit suicide because her exchange partner felt scammed, trawled through her profile for a post about wanting to flee her abusive husband, found the husband's linkedin, and threatened to send him the screenshot. That kind of freak shit is what you risk having to deal with once users have any kind of financial incentive to break social contract.

Not sure if it’s still a thing, but I remember freecycle being a useful site back in the day. Having a federated version of that which could also replace FB marketplace is a cool idea.

Shipping cost/effort makes this extremely not worth it for anywhere outside your local metro area where you can do in-person pickup. There are a good number of "Buy Nothing" groups scattered around the states. They mostly organize on facebook unfortunately. If you hook into your local anarchist community they almost certainly have some large signal group chats devoted to things like this. The anarchists are much more about trading of labor instead of just being a place to dump your old shit (trying to avoid the standard trap of mutual aid collapsing into charity) so there will be things like if you post your old stuff you will ask someone to cook you a meal in exchange for it. Sometimes this turns out to be much more trouble than just giving it away but it is what it is.

I think it would be really cool to displace the reliance upon Fedbook and similar closed platforms to create a Lemmy instance that offers this.

It would require a core group to establish though.

You could either start it with a group that does something like a produce exchange or labour exchange, or you could start it in a big city like New York or London.

You need a core group to get momentum and to reach critical mass, so if you tried to start it in a small city you would either struggle to get any uptake or it might kick off but it probably wouldn't grow beyond that little niche that you carved out.

It's also important to strike a fine balance between offering enough specificity without having a desolate wasteland where there's a thousand different comms, each inhabited with one or two users sitting quitely by themselves.