That's weird.
I have a theory.
(And no, all you Buffyites out there, it has nothing to do with bunnies. Though maybe...)

Clara was never at a signing with you. Someone got it for Clara and got it signed, to her. And Clara, following the instinct some have when given a book by someone else, never opened it herself, and just sold it to Dude, who also didn't read it yet because, yay!, it arrived *the same day* he learned that you would be signing in a Bookstore Near Him!