What is the difference between the categoryID elements under the Storefront and PrimaryCategory containers?
In order to help buyers find items they are looking for, eBay has a well defined category hierarchy. Categories have a name and also a distinct numeric ID.
Sellers are expected to list items in a category that best describes their item. Unless the item is being listed against an eBay catalog product, specifying the category in the listing request is mandatory.
eBay Stores sellers can create up to three levels of custom categories for their stores. Items can only be listed in store categories that have no child categories (subcategories). The category elements inside the Storefront container expect the numeric ID for a custom category that the seller created in their eBay Store, if any.