Go and check it out anon.
Now pop the hood or trunk and check the engine bay. Make sure fluids are topped off, if not there could be a leak. Check the block and head for any leaks and residue. if the engine cool and has been sitting for a while check the dipstick. Make sure there are no flakes in the oil and that there's no antifreeze either. Smell the dipstick. Should have a slight smell of gas but if it's too strong a rebuild might be needed. If it's a turbo check the turbine for any play And or oil residue on the blades Check the tail pipe for excess soot and or oil. Start the engine. Hear for any ticks or misfires. Now check the rest of the car and let the engine run.
Inspect everything. Get on your face and check the underside for rust, worn bushings, and dents on the undercarriage.
Then check the body panels, make sure the gaps between body panels are all an equal thickness. Any uneven gaps could be a sign of a previous accident. check for dings, dents, bondo
Now check the wheels, brakes, any visual wear on the rotors, tires, alignment. Turn the wheels all to one way and move the front suspension parts like tierods, control arms, etc. If they move freely or if you can hear a clicking some work will be needed.
open the doors and stand on the side steps and hop the car up and down. Do the same with the trunk and hood. Make sure there's no worn shock
If all's good take it for a drive. Check the clutch and gears. try to ride the clutch to heat it up. A cold worn clutch won't slip as much a warm worn one. Do some quick shifts and downshift into every gear. Also do random jumps like 1st into 3rd then 5th then down 2nd.
Get on the freeway and punch it.Make sure there's no humming or clicking Or vibration. Make sure the engine builds up nicely without hiccups or misfires.once you hit speed drop it in neutral. Don't brake just let it rest in neutral. It should idle to a normal 700-1000rpm without fluctuations or almost about to die. If it is theres a fuel system issue.
Now put it back in gear and check the steering and alignment. try to hit a couple of holes or cracks and hear for any suspension issues. If possible hit a speed bump.
Also if it's a turbo there should be a high toned whistle not a whine.
Go back to the guys house and park the car and leave it running. Pop the hood and check the engine no leaks or smoke now that it was running hot.
By now you should be around 30-45mins into the test drive and the car should've ran long enough for any codes to pop on if the fucker did try to clear codes before you got there.
If all seems good start throwing him offers. The key to negotiatiting is to not act desperate or entirely interested. Make it so he's the desperate one not you.
And bring someone with you. Sometimes the littlest things will slip your mind but the other person will take notice. And use common sense And your best judgement If get an awry feeling then ditch it and look for another.