I have a book I wanted to post but I discovered that I had highlighted two sentences in it when I read it a long time ago. This was probably because I read it as a part of a class at the time, even though it is not actually a textbook. Is this still not postable because of this? I thought I'd ask before I gave it away or sold it instead. :( 

You need to PM the requestor under the text book exception and describe the condition with the highlighting and wait for the requestor to acknowledge that it is ok to send or tell you to cancel the transaction. If you do not get a response, you will need to cancel as the requestor needs to agree.

Thank you!!

Another option if it is a wishlist book is to post it in the unpostable wishlist thread and describe the condition. You can still ask a credit for it. http://www.paperbackswap.com/Post-UNPOSTABLE-WISHLIST-BOOKS-Here/topic/117610/

I have done this a few times. send a message about some flaw in the book. sometimes they want it anyway. sometimes they don't want it. Best to ask.

oops

Yes, regardless of the subject you can post the book BUT ONLY by following the textbook rule.  Becuase many non-textbooks are used in the classroom, PBS has a rule for these situations. You MUST notify the requestor of the exact conditon, AND you must receive permission from the requestor to send the book.  You cannot send the book without permission from the receiver. 

Here's the official PBS rule for textbooks.

Just as a comment, I've followed the rule and sent a PM before mailing books with various types of writing in them, and I can't think of any that were declined. Most people want the book anyway, expecially if it's a Wish List book. None of my books that I've posted this way have had an excessive amount of writing in them though. Usually just a small handful of pages affected.

Well this book does have a few people wishing for it so I'll have to post it and see if any of them are okay with the two (only!!) randomly highlighted sentences in it! Thank you everyone for your input, it is very much appreciated. 

I have posted a lot of  plays, fiction and classic books used as textbooks that many would not expect to be 'textbooks'.  I let them know what is in the book, (highlights, underlines and how frequent they are )and ALL have been requested.