Doing a compression test on a car your buying privately? LOL Very good you gny turn up at the sellers house with your toolbox? Haha Ive heard it all now. Just be vigilant mate and if possible post up a link to the ad on here and im sure people will give you some assistance. Even better is to buy a car off the forum where the history is available on here like Matts. Main things are to start it from cold and have a listen to the engine for any knocking and check the rear for smoke. Check the oil looks good and not just black water. Check for signs of accident damage aswell. If I was buying outwith the forum I would be aiming for as standard an example as possible that way it hasnt been modified incorrectly.