Make sure that the owner has a full service history of the car. Every single piece of paper. Another thing a lot of people forget is to ask for the spare keys. If the owner can not produce the papers and spare keys, walk away.
Check every inch of the car, inside and out. Double check all the controls and electronic indicators. From the window controls to everything on the dashboard. Shut lines are usually a good indicator if the car was in a fender bender. Take the car for a drive and check that everything feels and sounds right. Dont just drive around the block. Drive at least for 10/15min so that the car warms up nicely. When you get back and park the car check after a couple of minutes if there are any oil spots on the ground or any other stuff dripping.
Because you are a new owner, you will have to register the car on your name and go through a roadworthy test again. This is why it's important that you make sure the car works 100% before you buyand take it for roadworthy. If it fails you will obviously have to pay for whatever needs to be corrected. You should visit your local traffic department who will help you with all the correct papers and stuff including getting police clearance if you fear the car might be stolen.
I hope the car is not one of these chinese rubbish. I would also not take it if it's a french or italian car. Bad resale and you will pay a lot for spares. Also stay away if it's a Seat. (They have stopped selling the cars in SA)
One last thing, also make sure the car was prefably purshased from new or had one previous owner. Absolutely stay away if the car had MULTIPLE OWNERS, is a DEMO MODEL or has a history in the RENTAL CAR market.
Good luck!
Anyone please feel free to add anything they know or experineced.