HDDs aren't a bad thing to buy used... like anything used there is risk for it to fail. That's part of the reason why the price is a lot lower than new. Ask for power on/use hours and SMART info to mitigate that risk.
Nothing wrong with used PSUs... it's not like they are like mining cards and rode hard to be only left for others...unless it's a mining PSU, but who knows. With everything there is risk. The good thing about PSUs is they are black boxes and not messed with, generally, so, I don't see too big of a concern buying a used PSU, personally.