Buying a used card isn't inherently bad, and can be a great way to save money. With that said, you have to remember that almost everyone selling a used whatever *says* it is like new, barely used, etc. That doesn't mean it actually *is* like new, barely used, etc. Furthermore, almost every 290, 290x, 280x, almost every amd card in general you see for sale on ebay was used for mining (meaning it was running 24/7 under full load, and may not have been adequately cooled as a lot of miners skimp on that sort of thing to save money).
None of this is to say you can't find a good deal there, just understand you are probably getting a heavily used card that may or may not have been taken care of. I would definitely steer clear of any brands that do not allow transfer of the warranty, or have warranty issues in general.