(A customer comes to the counter to borrow a DVD, I go into the back and get the one he wants, and all seems normal…)

Me: “May I have your card?”

Customer: *presents a bank card*

Me:  ”I mean your library card.”

Customer: “You mean I can’t buy it?”

Me:  ”No, you can only borrow from a library.  You can buy DVDs in the shop around the corner.

Customer:  ”Oh…. so I can’t buy it here? I have to borrow it?”

Me:  ”Yep.”

Customer: “I wanted to buy it.”

Me: “You can only buy it from shops.  Are you a member of the
library?

Customer: “No, I wanted to buy this DVD.”

Me: “You can’t buy things here, you can only borrow things when you’re a member.”

(By this point there is quite a long queue behind him, so I ring the bell for assistance.)

Customer:  ”What’s that bell for? Is it for getting a copy I can buy?”

Me:  ”No, it’s to get assistance for the other readers.  If you’d like to buy a DVD, I’d strongly suggest you go elsewhere. ”

Customer: “So I can’t buy it?”

Me: “No.”

Customer: *pauses* “Oh… I wanted to buy it.”  (After one more pause, he finally leaves.)

Related:
Stupidity Is The Mother Of Repetition
I Think She Wants A Discount