A thing we do in the Trading Forum of another place I post in is the rating system.
Every time a successful transaction is completed, both users PM a moderator to let them know. The moderator then writes their names in a separate thread in the forum. If the name is already written, each subsequent transaction adds a plus next to the name.
I think it would be useful. I know it is useful at the other forum I post. Adds to the trust meter.
I don't know if I can link to it, but here is the thread I'm talking about (you can jack some rules from it too

):
http://forums.modojo.com/showthread.php?t=68124