One thing to be carefull with when you're playing with the AH : if an item is listed at a given price, that doesn't necessarily mean that it will be sold at this price. Addons like Auctionneer record all prices whether the items will be sold or not.

My best advice would be to find a niche (there are tons of them, with very few vendors but enough buyers for them). The more competition the more risky market. Some people just buy reagents when price is low enough, and craft blue quality low levels items that sells well because they're almost the only one to sell them. Niches are different on every server.