Edited the post:
I remeber a while ago, there was a scam you do with enchanting. It doesnt matter if your mats or their mats, as long as you can do the enchant they want.
anyway, what you did was enchant their item, just before you both clicked trade you sat down (x) and hit trade. The trade would be complete. BUT the enchant wouldnt work coz you were sitting down and it would say "Cannot do that while sitting". So you would keep your mats but still get the money or whatever they paid in.
enjoy