I'm not sure if this is something that you can do easily or if it's something that you are even interested in doing. But I was wondering if you could add a rating/rep to users so people can know whether they are reputable traders or not. Over the past few months I have noticed a lot more of the CE's and Blizzcon pets getting traded. With those pets someone is always taking a chance on getting scammed (at our own risk of course and not really WCPs problem) but if we where to have a trade rateing/rep the chance on it happening would be lower.
You could make it so that each user can only +rep another user once or make it so they have to have 10+ posts before they can +rep another user. My ideas on how to do it are pretty bad but you guys here are great at coming up with fair and unique ways of doing things so I'm hoping you can come up with something simple that works well.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."