I think my favorite dumb question from my bookstore days was when a woman needed to buy a copy of the book for her daughter's class.
"I was looking at the beginning, and it says it's an abridgement. Do you have the original version by Morgenstern?"
Now, that's not the dumb question, because being ignorant of the narrative conceit at work here isn't all that unusual. So I explained that Goldman had made up the whole thing about the original book yadda yadda, and yes this is the book her daughter really needs, and now is when the lights go completely dark as she asks, "Why would anybody do that?"
"Do what?"
"Write about a book that doesn't exist. That's not very honest."
::blink:: "It's fiction."
"I think I'd better talk to my daughter first, maybe it's another book she needs." And she leaves.