Im no expert but heres what I can think of:
*Check pinkslip make sure its not salvaged
*Check body make sure everything is aligned properly such as doors and hood.
*Check for leaks, oil, coolant, etc.
*Check all the gauges and electrical components make sure they all work.
*Test drive it and check for unusual noises from the tranny or engine. Make sure car doesnt shake and steer to one side or another rapidly.
*Check oil pressure, coolant tempature any other gauges.
* If possible take the car to a mechanic to check it out. (If the seller is agrees that is)
I think he would go down on the price, offer him 3500 or less then you guys will go from there if he's negotiable.