Another member accepted my request for a book.  However, she told it she wouldn't be able to mail until a later date.  I agreed to late mailing.  The problem is that it was never marked mailed and I think it's rolled over to the next user who has not yet accepted the transaction. 

Does anyone have advice on how to handle this?

Is it prior to the date that person could mail? If it is, I would send her a PM with a big warning in the header something like **DO NOT MAIL BOOK XXXX, FAILURE TO MARK THE BOOK MAILED HAS CAUSED A SECOND TRANSACTION!**** And then explain in the message that when they did not mark it mailed, your request rolled to another member and they should not mail the book because your credit will now go to that person. They will need to repost their copy when they are able to mail it.

The other option is the send a PM to the person with the current transaction to see if they could hold off doing anything while you found out what was up with the first person, or cancelling your transaction while you find out and re-requesting if you don't get it. But all that is more work and waiting for the people who DIDN'T cause the mess.

I had this happen a couple times to me!

What Melanie said