What John should do is he should find reliable and relevant information; perhaps look up the information in the Kelley Blue Book.
He can't ask his friend because he may want to buy a different car, so his advice may not be helpful at all. A car dealer may want him to pay more than he should, so that wouldn't be useful either. His net worth will not help him reach his decision on how much he should pay for the particular car. So this Kelley Blue Book, which is used to compare prices for used cars is his best choice.
