get the car on the lift and check it from the bottom, if the car is rusted................check the springs too, check the engine, drive the car ur self and let the mechanic drive too.
check the engine oil...........look around the car if it ever had a accident or something.........find every little thing that happened to the car........hear if there are noise coming from the engine.....turn on the heat and the air condiation.......check if the car has a electri peoblom...... check the lights, open the hood and see if the battery is good, if all the wires are good, check the belt is good, see if some thing is dripping or leaking, check the radiator......
ask what is the reason he is selling the car......then buy the car and pick up checks