It's easy to avoid problems like the above person stated with a 10k deal. You can
a) limit the amount one person can buy at one time (this could increase over time as the person gained rating as a good buyer)
b) on all transactions over a set amount require proof of identification, phone contact, and the exchange of a physical item along with the virtual item. Have it delivered UPS signature required and tada, the person signs for the item you have signature proof he received the item and the buyer is no longer only buying virtual items you now have proof he received a clothes pin that he paid 10,000 USD for
And there others those are just two that I can come up with off the top of my head.
As far as the idea itself I think it's a good idea, and I would definately be willing to be a middle men so to speak and do item trades for you in any games I have access to.
Right now that list of games is..
World of Warcraft
Guild Wars
Everquest
Dark Ages of Camelot (Although this one is inactive so the trade would have to be enough to cover my reactivation costs, or I would need to know I'd be able to handle enough trades to cover the cost).
If I could remember my pass and account name I could reactivate SWG as well.
Also I've been thinking about playing EQ 2 if I decided to do that I could do it there as well
So there are a good number of games I could help out in hehe.