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.
Should I buy a used psu? What should I beware of? I am considering to buy a used Corsair cx 650m (used since July 2018)
No. Especially since if there is one thing single unit that can fry your whole system, then its a faulty PSU.
You dont know what you get. Maybe the previous owner was a wannabe tech engineer and rewired something the wrong way etc.