For me, second-hand hardware when it's safety equipment is a hard no. If it isn't a dealbreaker for you, you still really need a massive discount to make the risk worthwhile; not only is it a used power supply but you also don't get a warranty on it. If it's not less than half the price you can buy a new one from thet same budget line (or a 450/550 if you don'ta ctually need a 650) *or* your equipment is expensive I still wouldn't consider it.