A primary category on the selected marketplace must be specified for all listings.

Sellers have the option of specifying a secondary category for the same item on the same marketplace. If a secondary category is selected, the item is listed simultaneously in the two categories.

Refer to Retrieve Categories for complete information about retrieving the category hierarchy for a specified marketplace.

If you define an item and a problem occurs with the category ID specified, refer to Category ID Mapping for information about how to keep category data for individual items synchronized with eBay's category data.