When WL books are textbooks, and I pm the requestor to let them know about writing/highlighting. I state "please let me know if this is acceptable, and if so, I will mail by ........." 

So, if they don't answer the pm by the time I've commited to mailing the book, do I then cancel the order and let it go back to FIFO or mail anyway?

cheers, chele

from the rules it states that if there is writing in a text book both parties MUST AGREE via PM.. 

I would not send the book, send a PM to the requestor as the reason why and post it again. 

writing or highlighting or underlining on the text pages is NOT OKAY

• Exception: if it is a textbook or workbook, these are expected to have highlighting/underlining/writing
  • if you post a textbook/workbook, the condition must be described to the requestor in a Personal Message
  • AND the described condition MUST BE AGREED TO in a reply PM before the book is sent
  • This is the one of the ONLY three cases in which a PM exchange about a book is required before the book can be sent (the others are for travel guidebooks and cookbooks--see below).

You must cancel the order if you don't hear back.  If it's a book with other copies in the system, you can repost.  If it is a WL book, you can get stuck in a bit of a loop, vis a vis waiting for the person at the top of the list to acquire a copy, so that it can be ordered by the next person (which will start the process over again).  I would wait as long as possible before cancelling, though

As everyone else has said, you must get agreement. You can wait as long as possible to cancel, but I would make sure to do it yourself before the system does it automatically.