Depends on the brand of pump and who re-built it and for what reason. If "rebuilt" mean the cartridge was replaced, then it will be just as good as new. If "rebuilt" means simply the cover was removed, cleaned, valves replace, and re-assembled, then it is still a used pump. If the brand is good, however, I would not have an issue at all with used. As a matter of fact, my main pump came from a pawn shop with no history and it is still working strong. Then again, it is an American made pump and not an Asian import.