Reading your post again, you sold it on to someone who then traded it in? it may have been written in the book it was serviced when you had it, but they can't prove you did it. That could have been written in at any time by anyone after you sold it. Not saying that is fair but I can't see that they can prove who stamped that book and when.

Who is chasing you, the garage it got traded in at at the garage that the stamp is from?