Interesting. I think the following are Ok to replace with used auto parts.
compressor, alternators, camshaft, coil packs, sensors, cylinders, Brake drums, Fly wheel, CPU, CV joints, gauges.
But before actually replacing these parts make sure that the used part that you are gonna buy is under warranty and not expired. Check for any kind of breakage or leakage before buying it. Don't feel shy to ask questions to the owner before buying it. Its your Right!