Are you using the Advanced Sell Your Item form?
When you choose any of the Book categories, the next page asks for the ISBN.
However, this won't accept most non-US ISBNs.
So you can either enter the title and author and choose from the (wrong) editions that are presented, or if there is still no choice presented, go on to the next page.
When you get down that page to Item Specifics, it will ask for the ISBN number.
You can also use Not Applicable for pre -1975 books or others that have no ISBN.
You can also choose whether or not to have the pre-loaded ISBNs show. It is shocking how often the catalogue has the wrong information for a particular number, although this seems to be because some publishers (ACE!!) will use the same number for hardcover and/or paperback and/or trade paperback, even if there are changes like illustrators or different introductory material.
