For sellers; who have you sold to on this site before, could be used as a reference check.

Good idea as well or maybe some type of feedback along the lines of eBay - both for seller and buyer.  Simple rating system to avoid a verbal assault or limit it to a certain number of characters, again along the lines of eBay, short sweet and factual.  Then if someone wants to contact the party involved, they can do so via PM and keep things private irrespective of whether any comments are for or against.