Eventho the middleman approach sounds like the best idea, this just makes the middleman a target for scamming a ton of money. Asuming we are able to find very trustable users, which is not something easy, we can't take care of each middleman's security. MSN, forum account, PC, etc. Eventually someone gets hacked and a lot of money gets scammed.
I really dont see how this is gonna work, seeing a lot of ppl getting hacked and not knowing how it happened. If this eventually happens, it needs multiple validations and locks so there's no chance someone uses a middleman to fuck a trade.