I had a book (actually 2 books from a deal) marked as lost in the mail. My credit was returned. What if the books still arrive??? Do I send the sender their credit back or does it get given back when I mark it recieved in the transaction archive? 

The only reason I ask is because I've had books mailed from Hawaii before and it took almost 2 1/2 months before I received them, so if they were in deed sent I'm sure they will eventually arrive.

Thanks!!! 

If the books arrive, you can go into your transaction archive and mark the books received.  Then, your credit is deducted from your account and given to the sender.

PBS recommends you PM the sender if you have something go lost just to make sure it really got mailed.  Plus, if they put any kind of DC on it, ask them to check that.