I’ve been on both sides of the counter and I don’t find some of this funny. It’s true there are clueless customers who ask dumb questions. Some are really dumb; others are are inarticulate people with sensible thoughts they have difficulty putting into words… In either case, I’m not sure condescending sarcasm is the ideal response. People I’ve worked with behind the counter tend to treat all customers (other than ones they know personally) the way they treat their most difficult customers. As a customer, if I see a clerk treat someone like an idiot–even if that someone IS an idiot–I’ll go elsewhere next time. You don’t need to be a jerk to run an interesting, quirky shop.
Concerning bookstore prices, a friend was once on the opposite end of this scenario. He chose a book clearly marked $18 in the proprietor’s own hand. But when he took the book to the counter, the bookseller said, “That’s not the price…” He then consulted a price guide (this was pre-Internet) and quoted an amount double the marked price.