This means all old editions of that book are of no value in the used book market. The professor who last used that title has decided not to reuse the book or is no longer teaching at Gordon. The course the book was used for is not being offered this semester, in which case you may wish to hold onto the book and try your luck at the next buyback. Prices are based on perceived marketability of a title. The risk factors such as the age of a book's edition and its demand on campuses across the country factor into the equation as well.
Wholesalers also have various shipping and processing costs on top of the risk factors.
Keep in mind, they are a business. When a new edition supercedes an older edition, the old edition is no longer used, which gives it no resale value. It is wise to sell your books as soon as possible after you've determined you don't need them. The book is in unsalable condition. Examples include books with torn or missing covers, water damage, missing pages, excessive writing, etc.