Wowecon.com
This website is a fantastic resource, often left unused and unknown to many players. The people running it collect all kinds of useful market information for you: prices of items over time, prices on different servers, items that sell too low, etc. It's a fantastic way to learn more about your market! I can't stress its usefulness enough to you all. The more you know, the more deals you'll find, the more efficient you'll be in your money-making. So, go surf Wowecon!