I’m not sure this was good advice for the guy to change his HVAC compressor out himself, especially with a used on. I’ve had a few compressors put in and never had any luck with used or even rebuilt ones. New is the only way I would go. Plus they never talked about checking for leaks, replacing the dryer, adding the proper oil, cleaning the system, etc. All of these are necessary to avoid a very quick destruction of the new compressor if not done right. No one wants to spend the money but for HVAC work, I would leave it to a pro.