I would really love for someone to create a plugin like Auctioneer is for WoW. It scans the auction house on your server and keeps track of the prices of everything and after you scan often enough, you can find good deals on items and know how much items generally sell for. If I knew how to write a plug in I would attempt to make such a thing myself, but alas, I do not know how.
So please someone make one and I will be forever in your debt.