My general rules for used parts:
CPUs are fine to buy, except for specific known faults like Raptor Lake. If there's a problem, they just won't work and very rarely degrade within the lifespan before you'll upgrade in the future.
GPUs, I recommend only buying if it's still within warranty. Beyond that, it's all luck, no matter what the listing says. There's often great deals out there for old cards, but it comes with the risk.
Motherboards, never. Always buy new.
RAM, I always buy used because most manufacturers have unlimited RMAs, and it's easy to tell when RAM is the problem. It just requires patience.
Storage, new is obvious if you care about your data and privacy, but I do buy lots of old HDDs for dirt cheap to store games/porn which have somehow all worked fine.