There is actually a subsection of the forums called "MiddleMan Forums" - go there to find a middle man. Pretty much what happens is the middle man oversees everything that goes on - I believe everyone has a different manner of doing this, but it works out the same in the end. Seller gives "item" to middle-man, middleman informs buyer that he has received the item, and buyer sends the money to the seller. If it is item for item, both are sent to the middle man and exchanged from there (if I'm not mistaken).
The seller has to agree to using a middle man (and usually will) before one can be used. If the seller does not agree to use a middle man... be wary of the deal you make with them!
This is the middleman subsection:
http://www.sythe.org/forumdisplay.php?f=307
There are free middlemen and ones that cost - around 10% of sale (usually Official Middle's). Pick carefully!