Never sell a green or above item to a vendor, you'll earn a lot more by selling it at the AH. Before posting it for sale, search for the item to see if anyone else is selling it and what the going rate for it is. Don't just undercut the cheapest one in the list - if there's one that is way below the cost of the others, buy it and then immediately post it for auction at the going rate. Also, make sure to sell any mats that you find on the AH as well. Know which ones are needed by people looking to increase their profession skills. Search for those and look for underpriced listings. Again, buy them and immediately repost for sale at the going rate.

These simple techniques will help you to build a large income at a fairly low level. By the time I hit level 40, I had no problem buying riding training, a mount, and all the new skills for level 40, with plenty of cash left over!