There aren't too many items that can be purchased, used for months and then returned to the store for money back. Textbooks are unique. Selling them back to the bookstore is a great way to put some money back in your pocket at the end of the term. Excessive highlighting, underlining or other markings may decrease a book's value at buyback. Generally speaking, textbooks will be purchased until we reach our shelf stock limit.
Publishers issue new textbook editions quite frequently.
Ordinarily when a new edition becomes available, the old editions have little or no market value. While these types of books don't offer cash value, your book is far from being obsolete.