Well let's say someone had a Scaryface from Halloween but missed the Easter eggs from Easter and wanted to trade and vice-versa. They could trade. If one scammed, just comment on them and that'll warn others not to trade with them... ez.
I tweaked her idea enough; so I can fully take credit for it tyvm.

I believe the staff could check the pc numbers and check the chat logs between the two players and see if both drop the items, and the other one either takes it and leaves or they both have a actual trade. Based on the drops, chat log, and pc number they should be able to record who scammed.