Rollback trick from chinese traders

Just experienced a rollback trick that chinese buyers are trying on you
They will buy something from you, but once the trade is complete you will lose connection and when you relog the item will be gone but the roll back will cause you not to get the currency

Some chinese guy tried it on me, I got my item back and he got his currency back, then he tried again cause the trick failed. I just chose to report him and ignore him the second time he tried