Welcome, Donna. Here's a help doc on this issue. If you like, you can PM me with the ISBN, and I'll research it for you:
When I enter the ISBN on my paperback, the listing shows a hardcover!
Problem: You enter the ISBN on your paperback and the book Listing Preview says "hardcover".
This means one of two things:
1. Your book is an Advanced Reader Copy (ARC)
If your book is a paperback, and the ISBN brings up a Book Listing Preview that says "hardcover", please check your book to be sure it is not an ARC. If it is an ARC, it cannot be Posted for swapping at PBS.
Solution: You can offer this book to others in the Book Bazaar Discussion Forum, but you cannot Post it to your PBS bookshelf.
2. The publisher chose to assign the same ISBN to more than one version of this title.
Publishers sometime choose to assign the same ISBN to multiple versions of a book. However, PBS can attach only ONE booktype to the listing for an ISBN. If your book does not have the booktype assigned to the ISBN in the PBS database, you will need to post it with a customized listing, using the Post a Book Without an ISBN link on the Post Books page, so that the booktype matches your book.
Solution: You will need to post this book with a customized listing for the book, using the Post Books without an ISBN link on the Post Books page.
Solutions to common Book Posting Problems
How to post a book
What can be posted for swapping here?
How do I post a book without an ISBN?
About non-ISBN items at PBS