The prices for all kinds of things fluctuate. I talked a little about this in my post for casual auctioneers, but there is serious money to be made with market timing for very little investment. Basically, the goal is to watch prices for a few weeks, try to predict their next swing, and take advantage by buying low and selling high.
Easier said than done, like most things worth doing. You might get hundreds of items you'd like to watch, and short of a photographic memory or a manual spreadsheet, you're going to need some help keeping track of it all. Enter Market Watcher. This is a nifty addon that, once configured, allows you to scan the auction house for just the things you're interested in, and will show you a graph of their prices over time once you get a couple scans done.