Last year, I also sold one of the American Girl scene books, and I took it to the post office to ask if it could qualify as media mail. All 3 clerks that were working the windows said it DID qualify for media mail.

ETA: I was confused about it because some sellers were shipping priority, some were shipping parcel post, and some were shipping media mail. When I saw it, I didn't think it would qualify so that is why I took it in and asked. They all 3 said it did qualify. BUT, you know what? I never did receive feedback on it, and the buyer never would return my messages when I asked if everything was OK. I'm thinking the guys who told me it was OK were wrong.
This is just to add to what grlpwrd said -- even a previously used priority mail box can not be turned inside out and used again for shipping in a lower class. The box CAN be used again to send a priority item. Priority mail boxes are for priority mail *only* -- NO exceptions. Yes, it is correct that the newer boxes have USPS priority mail printed on the inside now.
Yes, sometimes media mail is opened to check that it is a media mail approved item. My dh buys lots of books on eBay, and we can see that some of the envelopes have been opened and re-taped.
I dropped a media mail package off one day in another town, and the mail clerk lady grilled me. She even asked me if I had written a note on the inside cover of the book to avoid a first class stamp.

Finally, I told her to just open the package and inspect the book. She declined and started asking me more ridiculous questions. I told her again to just open it and see for herself because that was the only way to
prove I was answering honestly.
I see where they are coming from though. I bought my twins a peek-a-block elephant last year, and it was sent media mail rate in a priority mail box turned inside out.

The seller charged priority mail rate though.
