In terms of the trust problem, one easy way to solve it would be to just require real names. Instance admins (maybe also moderators) must post an address, a name, and a (redacted) ID. A registered corporation would also work. Then, they would provide escrow, taking the payment but only giving it to the seller once receipt has been confirmed. The concern would be fraud on the part of the purchaser. There's no foolproof way to fix that, but if both buyers and sellers have "reputation" scores it would be pretty easy to tell if someone's lying.

The admin could also skim 1-2% off every transaction, and then put that into a fund to pay buyers in the case of complaints. That way both the seller and buyer are satisfied, and reputation scores can be used to boot probable fraudsters.

Either way, the system would also allow buyers and sellers to arrange payment in-person, in which case there would be no guarantee needed and the admin wouldn't take a cut.

This system centralizes power in a small number of people who can be sued. Everyone else stays anonymous, and if they're bad actors the admins deal with them. If an admin is a bad actor, their name and address is posted publicly for the world to see. Obvious problem here is that fewer people would want to be admins, but maybe it would be possible to set up a corporate structure where the owner's identity is revealed only if they're being sued -- I'm not a lawyer and you'd have to talk to one. Maybe there could also be a way for them to post records of every transaction in a verifiable yet anonymous fashion, to prove they aren't skimming anything off the top (beyond whatever they say they're taking for server fees).