I have several autographed test (and other famous) pilot biographies and similar books, a christmas card from Ann Welch tucked into the back of her autobiography, and a much prized signed lecture programme by Gene Krantz on my office wall, but none are for sale, nor will be in my lifetime.
Simple answer, people haven't generally asked for them in my opinion. That's not how pilots tend to be treated, even if they are relative celebrities. Also if people, like me, have asked for copies of books to be signed - they become prized possessions, not put up for sale or donated to museums.
G