Back in the day it was possible, for example, to announce a certain item on trade and when a buyer was found, instead of a direct trade the seller would pick any vendor gray item, wrap it on a gift box and COD to the seller. The seller would open the mail (and pay the COD) and when opening the gift, the item he wanted was not inside.
Was quite a common scam in Vanilla but no idea if wrapping items on gifts is still possible currently