i listed a book that was used as a textbook. Someone requested it. I pm'd them to tell about the underlining to see if it was ok to send and have not heard back in 3 days. They have not even read it yet. 

The date that I said I would mail it by has come and gone, but still some time left before the system cancels it.

what do I do? Just keep waiting until either I hear back from requester or it gets cancelled by PBS? 

Very frustrating that people don't respond.

You should cancel the request just before PBS cancels.

Whenever you post a book that needs PM communication, best to choose the longest possible mail date. That gives you time to get a response.

If they don't answer, you will need to cancel the request. I would wait to the very last day just before PBS cancels it, as well. The day that PBS will cancel it will vary based on how you picked your mail date, but it is at least 2 days after the day you picked, and could be up to 5 or 6 I think (it;s been a while since I examined that closely).

In any case, the cancel day/time is in red if you look at the transaction.

The guideline for this situation is in PBS rules...never send a book under the textbook waiver without permission from the receiver.  Without their permission, the book is unpostable and you run the risk of a RWAP and no credit.