Could you supply me with the basis for your assertion...
...that: "...the "book sale" section which is always a great indicator of what books the librarians dislike."?
Never having been a librarian, I have no direct information on how these books are selected. Do you have some particular knowledge about how this is done, that informs you that librarian preference is a guiding factor?
I would guess that age/condition of book and use history of book (not being checked out) would guide the decision. But I'm certain that Maryanne can fill in this blank.