There are different ebay requirements depending on what listing category you use. Its not simple. The UPC mandatory requirement was put in place some time ago. I put N/A in the box, it works just as Does Not Apply.

The codes returned by eBay seem to be nonsense, and are of no help in figuring this out. I import and sell products under my own brand on Amazon and eBay, so I had to purchase UPC codes. Amazon added the requirement first, eBay copied them.

A default entry would be handy, many of us sell vintage items, and some of the categories we use for old items require the UPC, and some do not.