A few days ago, I received a book that had no ISBN. This wouldn't be a problem, but the version of the book I requested did have an ISBN. I marked it RWAP and explained the problem to the sender, but have received no response. A day after the book arrived, after I concluded that the old, brittle book I had received was unusable (I had ordered the book for the library I run, and had expected the much newer version), I requested either my credit back or the correct copy of the book (in case the sender had simply grabbed the wrong one to mail).

I still have not heard anything. I received the book on Thursday, but the sender has not responded to either of my PMs. What do I do now?

Did you ask the sender to respond to you by a certain date?  If not, I would PM them again. Here's what I'd say: "As I mentioned in my previous messages to you, there was a problem with the book you sent.  The ISBN was not correct (in fact it has no ISBN, so was posted incorrectly), which is against the posting guidelines.  I am requesting a credit refund by X date (give a date).  If you'd like the book returned to you, I will gladly mail it back if you would like to cover postage with an additional credit.  I will promptly mark this problem transaction as resolved upon the return of my original credit.  I would appreciate your prompt response and refund, or I will unhappily mark this transaction as unresolved.  I appreciate your timely response."  If this does not get you a response, mark it unresolved and contact PBS via the Contact Us link.  Failure to respond to PMs is something they will address with the sender, though they will not intervene with regard to the credit refund.  Good luck.

Make sure you mark the lack of response in the RWAP followup in your Transaction Archive.  Then, you should send another PM, stating that PBS rules state that members must respond to problem transaction PMs within a week, and then politely give a deadline by which you expect to hear back.  Could be your member is just procrastinating on response, and a deadline may help spur them to action.

If they haven't responded after a week, contact the site administrators.  They will not intervene in a credit dispute, but they can verify the member is active and get them to give you a response.

Help Center says:

If the sender does not respond to your PM within 5 days, submit this information in follow-up from the transaction archive.  This will send an email to the sender, prompting him or her to log in and deal with this problem swap.  If the sender does not respond to that email within 2 days (a total of 1 week since your original PM), contact us.  Failure to respond in an active account is considered admission of fault.