I know that textbooks are allowed to have some writing but you have to PM the requester to confirm that they really want it. Recently, though, I've read that other books may qualify as a textbook if they were maybe used in a class. I have a book that has Book Club discussion questions at the end. The numbers -- and ONLY the numbers -- of some of the questions have been circled in pen. Can I post this and then PM the requester? Or do I just make it a freebie with another order?

Thanks in advance!

You can post it and PM the requestor about the condition. You must get a PM back saying that he/she would accept the book in its condition, or you must cancel.  It can potentially go in a loop if it's a WL book and the first (however many) people don't want the book -- you might have to ask them to put up an RC if you plan to repost in hopes of reaching a willing person further down the WL line.  

The reasoning outlined in the Help Docs:

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 PBS 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 PBS; BUT since the textbook exception in the Book Condition Criteria for 'Swappability' at PBS 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.

PBS guidelines are broad as to what constitutes a textbook, as many types of books are used in classrooms and learning situations these days.  DD is frequently looking for some classic or other non-traditional text she needs for one class or another.  I've received several books that have been used for this purpose also, notably biographies.