This great website (which accompanies a book) collects "Absurd Remarks made by Customers in Secondhand Bookshops," some of which are absolutely hilarious; others are just plain disturbing. I've heard at least a few of them in the shop, especially "You don't need a book, you've already got one at home," which seems to be a particularly (and most unfortunately) common message from some mothers to their children.
Another recent one was from a guy who came in asking for a book and kept calling it "really old, really old and hard to find." I asked when it was printed and he said "1998." There were several hundred copies online.