Actually, the way the system and rules are designed, it does not matter what the "objective definition" of a textbook is.  The club members decide this - as long as the rules in the Book Condition Criteria for 'Swappability' at PaperBackSwap are followed.  See the explanation and examples below.

If a member considers her book a textbook she may Post it with underlining/highlighting/writing on text pages, according to the "textbook exception" in the Book Condition Criteria for 'Swappability' at PaperBackSwap; BUT since the textbook exception in the Book Condition Criteria for 'Swappability' at PaperBackSwap require her to contact the requestor and obtain PM consent to the book's condition, she won't be in the position of sending a "stealth textbook" to someone who doesn't expect to receive one.

Examples/further explanation:

• Member A posts an Economics 101 textbook - something that most people would clearly consider a textbook.   

• Following the Book Condition Criteria for 'Swappability' at PaperBackSwap textbook exception rules, she sends a Personal Message to the requestor when she gets a request, describing the book's condition.
• Chances are good that she will get PM consent from the requestor to send the book with highlighting/etc. 

• Member B posts a novel (fiction) that she used in a class - something that most people would NOT consider a textbook.
  • Following the Book Condition Criteria for 'Swappability' at PaperBackSwap textbook exception rules, she sends a Personal Message to the requestor when she gets a request, describing the book's condition.
  • Chances are good that she will NOT get PM consent from the requestor to send the book with highlighting/etc.
  • If the requestor declines the book in its condition, Member B will have to cancel the request and repost her book.  It will go to the 'back of the line' for requests.  (If it is a Wish Listed book, it will be offered to the same wisher and the sender will again have to cancel.  She will have to wait for the wishing member to get a copy of this book from someone else before Member B can post her copy again.)

So, the farther the book is from a 'textbook' in common perception, the more likely it will be difficult to get consent to its condition.  Members in Member B's situation may encounter several declines, and will eventually realize that the book is not likely to be acceptable to the club in its condition.  Member B will stop trying to offer it as a "textbook"  and will consider it instead an unpostable/damaged book.

In this way, the club defines "textbook" on a case-by-case basis, and no one who is following the rules will be able to "surprise" another person with a highlighted/written-in/underlined book.