This is just out of curiosity, as the person and I worked it out, but it's never happened to me before. I posted a book to my bookshelf, copying exactly the ISBN on the back of the book, someone requested it, I sent it, and got a notification back that it was they had ordered an audio book!  I've never even seen an audio book in real life and have nothing but paperbacks, so I went to my Transaction Archive, and sure enough, it was listed there as an audio book.  I'm mystified...admittedly I didn't check real carefully to be sure everything was correct, when I typed in the ISBN and the right title/cover came up, I just clicked "post", assuming it was correct.  Needless to say I will check more carefully in future, but my question is, how could I type exactly the ISBN on the back of a paperback book, and having it come up listed as an audio cassette?  That's weird!

This would be due to the publisher. They have to pay for each ISBN. Some publisher's reuse the numbers.

I had an older paperback. I had ordered it, it was posted as a paperback. I received it and when I was done posted it. Then one day I noticed the listing had changed to an audio cassette.

PBS has a policy to be very difficult about changing the binding. A member basically has to prove that the book was never issued in the other formats. 

When I am posting a book by ISBN and notice that the binding type does not match I back out of the posting process and then Post it Without an ISBN. I then include the ISBN in the book description section to indicate that the book has an ISBN but could not be posted using it.

The help doc that applies would be "When I enter the ISBN on my paperback, the listing shows a hardcover!"