Ask him how often he changed the oil and fuel filters? How much towing did he do with it? When you start it watch for white smoke or anytime during the test drive! Pull the cap off the degas bottle and check for any signs of puking (any white stuff around the degas bottle) that's all I can think of right now! Make sure if you do purchase it change your fuel filters and oil and filter even if he says he just changed them only use motorcraft filters good luck!