Cpu and ram are probably the safest in my opinion. GPUs are usually pretty good, but verify fans functioning and avoiding old mined on cards being the priority (unless priced appropriately). Motherboards have more that can go wrong, and ssd die with use, so they are what I am less likely to get used. That being said, I have no issues buying them used either if the price/situation make it worth it. Power supplies I would buy used if it’s a budget build, but only if it is a quality unit and not too old.