Everything is negotiable when buying a used car. Sounds like to me the tires were not properly inflated. Besides the way you drive will determine how many miles you get from a set of tires. The rears will last about 1/2 as long as the fronts. You cannot rotate front to back. I have 11k on my tires and fully expect to get another 15k out of them. That equates to about 3 years or so for me. As one poster said it is part of the deal with ANY performance car. When the NSX came out owners were getting 5k out of a set of tires. The next year they changed the camber/toe-int and things improved alot.