I just received this:  This book was sent to CA on the 29th of November. It shows as canceled. I do want to receive credit for this book

Then she gives her name & address

I double checked and sure enough, this book is already marked mailed, on Dec. 3rd by someone else.  Which means I've already spent my credit on this book to someone else.

I can't afford/don't want to pay twice for the same book.  what's the most polite way to do this? I do feel for her but don't think there is anything I can do?
 

You can let her know that because she didn't mark the book mailed, another copy was automatically ordered and is on the way. Telling her flat -out that "I'm sorry, I will only pay a credit for the second book" will help her to remember to mark her books mailed.

And please try to remember that people are supposed to pay for their own mistakes, not you.

The Help Center says -

If you received (or will receive) two copies of the book, you are not obligated to give a credit to the sender whose transaction was canceled.  If you got (and will get) only one copy, you should mark the book received so that the sender gets credit.

http://www.paperbackswap.com/help/help_item.php?id=138

I'd reply something like, "I'm sorry to hear that you've sent the book on a cancelled transaction.  Unfortunately, another sender has already mailed the book.  The Help Center says that I'm not obligated to give you a credit since I'm getting a two copies of the book." and include the link above.

Make sure when the book arrives that you are marking it received from the person who did not forget to mark it mailed! You don't want to inadvertently punish the person who did nothing wrong!

The active transaction on your My Account page is the one you'll mark received.  It's for the second book.

Since you asked for "nice", you could say something like, "Oh that's a shame!  When you missed the deadline for marking it mailed, the server automatically ordered a copy of the book for me from another sender.  If I had received your note before the second member had printed out the wrapper, I'd have canceled the second transaction, and then would gladly have marked your book received when it arrived.    But I only had one credit to spend on this book."

Ooh Patty, great reply!

Another option, although you're not obliged to do so: If the first book was mailed first class, you could try to catch it before opening it and mark it 'Refused - Return to Sender' and give it to your carrier. First Class packages are supposed to be returned to the sender at no charge. If it's media mail, it may or may not be sent back to her and she might have to pay for return postage.

Thanks for the replies.  Patty that was exactly what I was after "nice" :)

Oh good!

BTW, Lsressler, refusing the package wouldn't be fair to the second sender.  He'd be paying postage to mail a book, marking it mailed on time, and not earning a credit for it. 

BTW, Lsressler, refusing the package wouldn't be fair to the second sender. 

Maybe I didn't explain it clearly but the second sender wouldn't be be out their credit. I advised as an option that she look out for the FIRST sender's name and address (the one who missed the deadline to mark the book mailed) and refuse that package. If it was sent first class it would be returned at no expense to the first sender. She'd mark the second sender's book (the one who did mark the book mailed) received. The first sender who didn't mark it mailed on time would get her book back but no credit.

Refuse the book from the one that had the transaction cancelled and it will go back to the sender, who will have to pay the return postage at the PO in order to pick it up. No return postage on Media Mail rate,,and she can have her book back .

I don't think anyone is suggesting that the second sender not get credit. But if Cindi doesn't want two copies of the same book, offering to refuse the first senders book is something to consider. I'd run it by the sender first because personally, if I wouldn't want it back, even if I was out a credit. But then I think I would have worded my PM a little different than saying 'I do want to receive credit for this book'. Maybe the first party doesn't understand that the reason it was cancelled was because she didn't mark it mailed.

Edited to correct typos

"Maybe the first party doesn't understand that the reason it was cancelled was because she didn't mark it mailed."

This was what I thought too. Patti's "nice"answer does include this info - but, the original sender needs to be made aware this is her/his fault - so, while she may want her credit for the book, it is due to her not marking it mailed that "that ain't gonna happen"  :-)