Sorry to hear this happens but I think this tactic is not new.

The only thing I can suggest is for the seller to insist on a copy on the bank in transaction as proof of payment and then insist on the buyer to bring the original copy as proof during the COD/hand in of items. This way, the seller won't hand their item to the wrong party while the conned buyer may get back their money provided he/she can proof that he/she did bank in the payment. The only thing is if the buyer or seller are from another site, then it's difficult to trace.