1) Always offer something cheap like stones, in exchange for something expensive. ie if you are giving some one a +1 item to loom for you get them to put up stones for the item on the market and then you go into the market and accept the trade. If you do it the other way around you will get jacked and really only have yourself to blame.
2) Make sure you can trust the person your trading with. If you've never heard of them and don't know who they are friends with then your putting yourself in a jackable position. Try and find out who they know and who they have traded with. Get them to send screen shots of their char screen and equipment screen to validate thing they are saying like "I'm gen 3 and want to sell this loom point to do more trading in the market." Obviously a photo can be shopped, but seriously, no one around here would go to the trouble. What I mean is that every one is fat and lazy, rotund.
3) If you are doing a trade and have a mutual friend that can be trusted get them to escrow the trade, that way you have an "uninterested" 3rd party. This point is far less important than the first 2, and you can avoid it all together if you know they are reliable.