Some tips for buying off Craigslist...

Keep in mind any excess costs you may need to put in when buying a bike. Does a bike need a new chain, bar tape, or tires? Does it also need to be recabled? This can easy cost up to $50 on top of the actual bike cost. If this is the case see if you can negotiate the price and mention you'll have to spend money to get the bike running.

Stay away from rusty bikes, unless it's just a little bit of surface rust. This usually means the bike has been kept outside.

Check the bike thoroughly. Make sure the wheels are true. Check to see if any components have been replaced.

Buy a bike only if it fits or you can resell it easily for the same price. No need to spend too much on a bike that doesn't fit you.

If you can, be patient and check Craigslist a lot. I've gotten great deals on CL by being patient and quick to respond to listings. My best deal was probably a Trek in great condition that I bought for $125. I spent about $20 on new cables and bar tape, put a few hundred miles on it, and sold it for $250. I also have a nice Bianchi I got for $100... I still have it as my commuter bike, but if I ever were to sell it, I bet I could get about $250.