May I suggest to NEVER valet your car. I never do anymore but my first car was my dad's 1968 Mustang GT, I had it valet parked at a mall in Scottsdale, when I came back, I found myself escorted to my car, when I looked inside, I saw the ignition switch (part where you put the key into) lying on the carpet, I had to start the car with a long piece of metal lying on the ground in the parking lot by reaching in far enough and twisting it to start the car. I think they should be more selective at least in hiring these valet's. As I was trying to find something to start the car with, I heard the sound of a clutch burning up only to see a Porsche 911 come around the garage, and speed up to its owner.
As for letting people drive my car for something in return, yes, I let my friend drive my NSX around an empty parking lot in Chandler, and it was implied that I could take his brand new Yamaha R1 bike around and tear it up. (By the way, I love the sound of my NSX when I am a bystander). I usually will let friends take the NSX around the block if I know they are capable of driving it (which is not too many people).
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1992 NSX Red-Black 5-Speed