The guy i bought my skyline wouldn't let me test drive it until i had paid for it, but that was cool, he drove me around noth sydney etc...
as a passanger it is easier to listen to squeaks etc... u get a good feel of the car and u can tell how the previous owner dove the car.
also someone who is interested in buying your car wont mind if u dont let them drive... if they do then i doubt they are really interested