Yeah that's happened to me. I wanted a Clefthide Leg Armor, and gave him the materials. He immediately logged off, then 5 minutes later he logged back on and said "Nah just messing with ya". Then he gave me the Clefthide.
This guy was a friend of mine, so when he asked for lifestealing enchants, I took his mats and didn't give em back. >.>
Better to be thought of as a fool, then to speak and remove all doubt.
What i do is give them the mats and they give me the money value of the mats, that way if they run off with the mats i still have money to buy it off the AH. Then again, i have friends of every profession that i trust will not gimp me.
::Dips pistol into mountain of whipped cream and licks it off:: Mhmmm... Pistol whip... ~ Spen