Narrow down what you want price wise and set up an RSS feed of all of those results from the craigslists, kijijis, gear swaps, and other websites to your email morning. If there is a good deal out there, it won't last long.

Bikepedia is your friend. A lot of ads don't list model or year, so it's hard to tell what you are buying from the one blurry photo. A good deal might not be apparent if you don't have all the info. The more information you have, the better off you are.

Get an idea of what kind of rider the seller is. Did they buy it new and just ride it on nice weekends, daily commuter (rain and bad conditions can put a lot of wear on bikes) or are they a professional bike flipper that's turning a profit? Usually you don't even need to ask, a quick conversation gives you an idea about what the seller knows about the bike and what kind of person they are.

Give the bike a good look over. Check for loose or rough bearings in all of the rotating parts. Check the chain stretch with a rule. Check that the shifters and brakes work. Often times used bikes means broken shifting. If they are simple fixes, they often devalue the bike for people who don't know how to adjust derailleurs. Check for dents or bends in the frame. Gouges and scrapes on the shifters and handlebar tape are a sure sign of a crash.

Oh, and make sure you get a frame that fits.