Just listing a batch of comics this morning and put barcode from one comic only into required field and left ISBN blank and got two errors at top when checking listing. Cannot find a ISBN on any comic so actually typed 'does not apply' into the field where the numbers would go. There is no drop down list for me on the ISBN. The letters are light grey as I typed. Then selected 'Does not apply' from drop down list for MPN number and next stage of listing was clear without any errors or reminders about fields required etc.

Of course listing is not live yet (scheduled for tonight) so cannot tell if successful until that happens but hoping that this has fulfilled all requirements for new MPN numbers, especially when it comes to a batch of comics, books etc where there is more than one isbn or mpn.