If you're concerned about someone ripping you off, before you even open the trade window ask them a "confirming question" in /w. An example might be:
"So I'm trading you x, y, and z items, and then you're going to use them to give me X enchant, right?" If there is a fee involved, include that at the end. "When you do the enchant, I'll pay you <amount>." If they don't answer in the affirmative, don't give them your mats.
That doesn't guarantee you that someone isn't going to try and run off with your mats, but it does document the agreement so that if they do try to scam you, it becomes very easy for a GM to sort everything out and get your mats returned to you.