If you know what you're doing it can work. I bought an 18 month old Cervelo P3 TT bike with Dura Ace 7900 for about HALF of what a new one would have cost me in the store @ the time. That kind of purchase makes sense. Buying some no name Chinese generic CF bike, not so much.

But to have this work, you must have a clue. Know something about bikes, what size you need, etc. Otherwise, stick to the LBS.