Some say NEVER buy used gear! Ive bought used gear many times, for Cars and other things, there has been some bad expereinces but for the most part everything turned out well and i saved some money.
If yourgood with wrenches and have a facility to work on your car then its worth it cuz if sumn goes wrong or your missing parts you can always make it work. If you just want to have the car installed trouble free, then get a pro shop to install a new unit and you will have much less to worry about.
Piece of mind is worth some extra cash but if you have the time and the patience a great deal can be found and made to work.