I mailed out a book on 3/26.  On 4/21 it was marked received.  Today it showed up back at my house, marked undeliverable by the post office.  I'm guessing I sent it to a member with an inactive account which is why I got the credit even though the book was obviously not received.  Do I have any other obligations as far as contacting the member who it was supposed to go to?  Or can I re-post right away?

Browse the help docs for answers: 

http://www.paperbackswap.com/help/search.php?terms=a+book+i+sent+was+returned+to+me

http://www.paperbackswap.com/help/search.php?terms=a+book+i+sent+was+returned+to+me#what_to_do

You can't assume you got the credit because the member has an inactive account.  You can't re-post right away.  You need to figure out why it was returned and follow the procedures in the help docs.  Sometimes the post office is completely at fault.

Did you check the other members account to see if it's open or suspended? 

If still open, you probably want to contact the requester regarding the returned book.

If the requestor does not respond within about 5 days, you can ask PBS admin if they know why it has been returned and why you received credit for it.

BTW - some members mark books received by accident, or on purpose if they know they are out of town for an extended period.  If they moved and forgot to have mail forwarded (or some other error on their part) they may have marked incoming books as received.

I have had the exact thing happen to me: Member moved without changing their address at PBS, assumed that their mail would be forwarded, but instead they received a notice from the post office to come in and pick up their package (book I sent), they opted to not pick it up but marked it received. The problem was that I did not know any of this and when the book was returned to me as undeliverable, it was postage due...I could have opted to not pay the postage due and let the PO keep the book, but I was so curious what book it was, because I knew all the outgoing books had been marked received so I paid, discovered it was a book that had been marked received and when I contacted the member about it they got quite nasty. It all could have been avoided if they, the recipient, had let me know what was going on...that they had marked a book they did not receive as received. Or better yet, not requested a book when they were moving...or at least updated their address.