Yes, as you and hard2kill said, it needs to be clear that it is trade at your own risk. I think a simple forum would work, but we may want to put limits on who can participate such as someone who has been a member for so long or has so many posts. If someone wants to get involved immediatly we may want to require them to provide a third party feedback such as eBay.
If you could, code some sort of trust score where after a trade you can post a positive or negative and this would be displayed in our profiles or under our avatar.
Also, we may want to require that trades have delivery confirmation or tracking that way there are no hurt feelings if it gets lost in the mail, which isn't too uncommon. I would hate for some good members of this board to hold a grudge against each other over something that was an accident.