I'd like to return this book...

Lady: I’d like to return this book and get another one to read.

Me: Okay, we don’t do trade. We just buy books. [I look at book] Okay, it would be this much for that book.

Lady: Oh, I paid more for this book. See? [She points at our original price.]

Me: Um, yeah, we’re not the library. Why would I pay you what you paid for the book? We need to make money here…

Lady: Oh.

(The day is still early. I’ll keep you posted on further idiots…)