in general these are questions you should ask eBay. not the GarageSale user group.
But you can revise the listings as usual in My eBay and add the MPN (or “Does not Apply”).
eBay does always show an end date in GTC listings. But they will be extended once they will reach this end date. Maybe you have to look through your listings and add the MPN where its missing. Or you have to ask eBay how to find these listings.