After reading about Souljah's scam and other people's bad (or good!) selling experiences, It'd be a good idea to have a feedback system. On your profile, you could have a part where people you sold to or bought from can put 'good seller' or 'scam artist' or something. Or even have a list of people's names who are willing to sell/buy stuff (pm people to ask, and pm a mod to get your name on the list in the future) and then have 5 stars for a great seller, 0 for a bad seller. The ebay system would do wonders here.