
Introduce an auction system.
It could work like this:
-Seller puts his/her item in an auction box with the following input:
--Starting price
--How many hours the action will be up
--Eventualy a buy at once price
--Eventualy a (secred) minimum price (item wont sell when the ending bid is lower)
-Bidder sets a starting bid and a (secret) max. bid. The max. bid's gold is set apart for the duration of the auction. Whenever someone offers more gold your bid will automaticly be raised until your max. bid is reached. Eventually the bidder gets an ingame messege.
After the auction:
The auction program simply transfers the gold from the buyer to the seller and changes the items owner status from the seller to the buyer; The buyer opens the auction box and the item is there waiting for him/her.
Personalized items cant be traded.
Of course the normal ingame trading will still be possible.