I ebay and have had good results with it.

You do need to know what you are bidding on. From my experience, the sellers usually respond pretty quickly about any questions you have.

And you need to do it right! My first experience was when I was looking for a "ride around town" bike and went on ebay. Noticed several underprised Trek bikes and bid of FIVE of them. A week later I get this e-mail saying I was the winning bidder on one. With some bit of haste, I checked to make sure I wasn't going to have to explain to my wife I had just purchased five bikes. Turns out I was outbid on the others, and got a nice (probably never ridden) Trek XO-2, fully equipted for touring with racks, panniers, etc., for the minimum bid of $550. The panniers still had the sales tags on them. So it turned out very well.