I’ll buy almost anything used really. Well besides hard drives.
Power supplies are largely fine as long as they’re not really old. GPUs I don’t like buying big ones used. More just out of PTSD from the mid 2000s solder issues. Smaller (adequately cooled) does seem to have less issues.
Ram almost never fails. CPUs rarely fail. Motherboards are generally solid. I’ll put a used SSD into a machine I don’t care about. Cases basically can’t fail unless you destroy it on purpose. Air coolers don’t go bad. Liquid coolers will eventually need maintenance, but as long as they’re not really old should be fine.

