The barcode is needed to reliably identify items. In most cases, you can find it on the back cover of books. Books can also have an EAN (or ISBN) number. Books without an EAN, IBSN or barcode cannot be sold.
Books
If you cannot find the barcode on the back of your book, try looking on the first or last page of the book to see if you can find the ISBN instead.
CDs, DVDs and Blu-rays
If the barcode is not on the back of the casing, it might be inside the booklet.
Games and software
In most cases, the barcode can be found on the back of the casing.