Originally Posted by
Covert
Oh that's easy.
Personally I go for high quality armor scraps. If you stalk the auction enough, chances are you can find a stack of high quality scraps for around half of what they can sell for (generally they sell for 10g a piece).
If you're feeling risky you can try to pick up a mini sand demon pet for 800-900 gold and resell for around 1300 gold but keep in mind that this is risky, although I've successfully resold each time. :)
The market is very fickle so you'll need patience. Good luck!