I've noticed my book being sold as "used" on some websites but I have not received payment. Why haven't I been paid?

Often, retailers list books on their websites and advertise that they have ‘new and used’ copies of books. Retailers use this as a marketing tool to entice the consumer by frequently offering a small discount on the used version. This does not necessarily mean that they have an actual copy of your book on hand, which is most likely the case if you have not been paid a royalty for the book sale. If any books are sold to a retailer, you will earn the retail royalty amount which was agreed upon.

If a retailer or third party actually is selling a used copy of the book, you would have already been paid in the past for the original sale.