The professor's basis in the car is $$30,000,$ which is the amount of cash he paid for it$.$ For personal$-$use property, the basis is generally the cost of acquisition. The fact that the seller was asking $$40,000$ or that the Kelley Blue Book value was $$35,000$ is irrelevant for determining basis. Since the car was purchased for personal use rather than business purposes, the basis is simply the amount paid.