I have been in contact with the person who violated my requester conditions and they want me to send them the book back before refunding my credit.  It is my understanding that they should refund my credit first and if they want the book back they should send me money for it or another credit to cover the costs. 

When this was brought up the sender, they questioned my requester conditions, asking if it was neccessary.  I guess I think its not the point. 

Any advice on how should I proceed, since they are no longer responding to PM?

You may be at a stalemate if they won't refund your credit and give you an extra one to send it back to them (2 credits total) before you send the book back.  Everyone is entitled to RC and they clicked that they could meet your RC.

If they won't send you the credits to ship the book back to them and are no longer responding, contact R&R and share with them what is going on.  You may be out the credit and have a book that doesn't meet your RC but would it meet someone else's?  maybe you could relist it? 

I would try PM'ng them again, and remind them of the new procedure which helps PBS track people that do not work with the other party to help correct a problem transaction. This may encourage them to respond to your PMs. Maybe not, but it's worth a try.

People who ignore RC's suck.