I think Ebay can be a great place to get a cell phone -- you just have to be careful. Before buying anything, you should do your homework and just shop around a little ... it's quite easy to get ripped off on Ebay.
It's never happened to me though. I've bought several things through there, including two cell phones.
One phone said it was new, but it clearly wasn't (it had phone numbers stored, pictures saved, linked up to someone else's voice mail, etc...) It wasn't too heavily used though, so I wasn't upset :S
The second one was in PERFECT condition... I just got it today!!
P.S. Make sure you read the ENTIRE description, including the links added.