Well, I work in a used bookstore, so the usual is people who offer us about 50 cents for a highly collectible paperback that is worth 10 or more dollars... Then they get all pissy, and accuse us of being crooks, and usually stomp off in a huff.

(There's a paperback copy of "The Man Who Fell to Earth" in the back of the store that I want, but it's a first edition, and is worth 20 bucks. *sigh*)