Two issues I see with buying a used PSU:
1 - You have no warranty on a used piece, so if something goes wrong your cost may very well be money thrown away. Many of the manufacturers will stand behind their product, even to a second owner. You can ask around to see what sort of assistance you will receive (if any).
2 - The seller is pretty key. If the seller is honest about what he's selling and has taken care of it, there probably isn't any issue. But how can you really guarantee an honest seller, especially online?