What is the policy when you accidentally mark a requested book as 'received?'  I ordered a similar book (about the same television show) from amazon.com and when it arrived in the mail, I went upstairs to log in and check that my book had arrived (I always do this as soon as I open the mail so as not to forget!).  But later I realized that the book came from amazon.com and not from the paperback swap group.  In fact, I am still waiting for the other book to arrive, but it has already been marked as received and removed from my home page.  So what should I do now? 

Basically, you have no recourse at this point.  By marking it received, you have acknowledged that the transaction is complete.

If you marked a book received accidentally from the En Route to Me tab in My Account:

• This book was marked mailed; chances are that it will arrive.
• Please wait for your book; most likely it will arrive and you will have already marked it as received, and won't have to do anything further.
• Please try to be very careful about which books you mark received

Scalta nailed it.  I'd suggest sending a PM to the sender, explaining what happened, and telling her you'll send another PM when the book actually arrives.