I was checking on the status of a DC book I mailed and discovered it was marked refused. I have no idea why, I have heard nothing from the person who ordered the book. I downloaded and printed the label and mailed it media mail with DC. 

What happens now? Does the post office return media mail items to the sender? And will the post office want me to pay the return postage since it was media mail? 

It's very possible that the book will arrive back with postage due, return postage is not included in Media Mail rates.

Not sure I've seen Help Documents for this type of event.  I'd say wait a couple days to make sure it's not an incorrect USPS scan that gets correct to 'delivered' in a day or so.  Then I would probably pm the requester and very nicely ask what's up.  If they deliberately refused the package (I'd be hard pressed to think of a legit reason just now), I'd be tempted to forward the transaction to TPTB.  Moved, didn't want the book...at this point, I can't think of a legit reason not to accept and properly log the book in.

It may not have been refused.  I had a book coming to me last month that showed as "refused" when I looked up the DC number.  I've never "refused" a book.  When I checked my PO Box later that day, the book was there waiting for me. 

I also wondered about that Jean (incorrect USPS scan).  Which is why I recommended waiting a day or two, to see if the situation will sort itself out.