Thanks for bringing up these important points. My only thought as to a solution would be a flagging control system threat. So, perhaps users must have active accounts above a certain reputation in order to use the buy/sell service. So if the transaction doesn't go well they have something to lose when my collective of dolphins and whales flag down their posts and add them to a list warning other users not to follow or upvote them.
I don't know. It's a tricky one for sure. Don't like the idea of 'punishing' people.
Hopefully we will find a system that works without needing this threat.
Great answer!
I am definitely interested in this and will be following you, that way I can see any updates and also see what people are selling!