Quoted from playnoevil
"Basically, this is a classic transaction processing failure. One player transfers something to a second, the second receives the item. Before the central database can update the first players inventory to debit the item(s), he logs out. The closing portion of the transaction doesn't happen."
http://www.playnoevil.com/serendipit...-Warcraft.html
This is untested, but I would love if someone would test it.