I’ve noticed some celebrity book talks where admission is charged and a copy of the book is included in the fee, so nobody leaves without buying a book.

If the author is not a celebrity, I expect some people attend to see if it’s a book they would enjoy reading. I don’t think they should feel obligated. These events sometimes attract people who can’t afford to buy books and are just trying to get inside for a while on a cold day. They are sales pitches. Don’t blame the prospective buyer if your pitch falls flat.