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.