Most important question to ask is how long they’ve owned the vehicle. If they’ve had it for less than one year, walk away; it’s probably a curbstoner – a slimy used car dealer who operates off the street. If this is somebody with more than one vehicle for sale, that’s almost certainly what you’re dealing with.
Second, ask to see the maintenance records on the vehicle. Ask to have a mechanic of your choice look the vehicle over at your expense. If they don’t agree to this, walk away.
Ask to see the title. If they don’t have it, walk away.
Ask them why they’re getting rid of it.