Listing shows quantity '1' presently; can't edit that to any other number. It removes the '1' but only gives a pink box with message "Please enter a number". Can't put the '1' back in, either. Can't publish. The only escape is to "Return to listings"...
Have tried from shop item page 'edit'; from 'listings' page item-edit and item-gear-edit. All the same result.
Either this was a Firefox-related issue or it has been resolved. I changed the quantity on Chrome, then checked again on Firefox and it works there, too.
Problem solved, it seems...