Can you make it so that the first X times they log in after registering, there is a pop-up that says "READ THIS READ THIS READ THIS" telling them about the proper way to manage trades/loans/etc? I know there will be some who just click through it but if you make it blatant enough you can catch the majority and then those who don't care ... meh, they make their own bed.
Maybe also include in those instructions that they should report anyone who is trying to finagle a trade without using the form?