Just be prepared to get what you pay for. I know several people who have bought used turbo's on Ebay and got EXACTLY what they paid for - turbo failure. Of course, oil residue inside the turbo can be an immediate sign of bearing or seal failure. Shaft play is also another sign. Every turbo has a little up and down, but like WCS said in-and-out should be reserved for the bedroom.
Another question you need to ask yourself - what is my plan for this motor?
If you're going to use it as a daily driver and not run the boost up very much, a smaller turbo would work great. If you're going to pound it at the track and make some real power, don't cheap out on the turbo and get a good one.