As you might know, with the new patch engineers can make Epic Bullets / Arrows with 91.5 DPS. (Iceblade Arrow / Shatter Rounds). The matereals for them are really cheap - 2 crystalized shadows for 1000 arrows. The AH prize is usually ~15G on my realm.
To make tons of money from them you don't even need to be Engineer to make them. Go and buy 1 stack from the AH. Than using auctioneer, put like 15 stacks from 1 arrow / bullet for 8gold per stack @ the ah.
You have no idea how many people don't notice that they will buy only 1 arrow for 8 gold, instead of the 1000 arrows that they expect When they see the price so low they will rush and buy the items without hesitation. I've just made couple of hundred easy gold from 30 of those arrows.