The term BOOK SELLER can be broken down into two crossword solutions: AGENT (5 letters). An AGENT is a person or company authorized to act on behalf of another person or company, such as a book seller. A book seller is someone who sells books, either in physical form or online. They may work for a bookstore, library, or other organization that specializes in selling books. They may also work independently, buying and selling books from their own personal collection.