Buyers often browse online stores by "drilling down" through a hierarchy of product categories, so every item listed on an eBay marketplace is assigned to a category. Each eBay marketplace provides its own extensive set of item categories. Some categories are common to many marketplaces, and some are of interest only to a region or culture served by a particular marketplace.

On eBay marketplaces top-level, or "L1," categories are named groupings of similar items, such as Collectibles & Art, Fashion, or Electronics. Categories are then further organized into a hierarchical structure that is conceptually similar to directories in a file system.

All categories that contain actual listings are called Leaf categories. As an example, Collectibles is a Level 2 (L2) category, while Presidential is a Level 4 (L4) Leaf category within the Collectibles & Art hierarchy. Although the full path within the hierarchical structure is Collectibles & Art > Collectibles > Autographs > Political > Presidential, this leaf category is displayed on as Collectible Presidential Autographs.

When a seller lists a new item, the seller must specify in which category the item should appear. Similarly, when relisting an item the seller may specify a category other than the one in which it was originally listed. The category that a seller chooses for an item can dramatically affect how easily potential buyers find it.

It is important that a seller place an item in the proper category as it should increase traffic to, and sales from, the listing. In eligible categories, sellers can list items in an additional, "secondary" category. However, additional fees may apply when listing items in two categories.

Each category has a unique identifier for such purposes as listing an item. An item must be listed in a primary category, and some items may also be placed in an additional/secondary category. In most cases, the seller will choose as the primary category the category in which they think buyers would most likely browse or search for the listing. On some marketplaces, certain categories are required to always be the primary category if they are used. For example, when listing a ticket for sale on the US marketplace, a leaf (listing) category under the Tickets and Experiences L2 category must be selected as the primary category. For additional information, refer to Category IDs.

To list an item on eBay or to search for items in a particular category, the user of an application must specify the unique identifier of the category (e.g., for AddItem, use Item.PrimaryCategory.CategoryID,) and the selected category must exist on that eBay marketplace. To help the user select a valid category ID, an application can download information about the category hierarchy which includes category IDs and human-friendly names, and then present that data to the user.

Some listing features are available only in certain categories. For example, a seller can specify a reserve price for an auction listing, enable Immediate Payment, or list with Parts Compatibility data for a motor vehicles parts and accessory listing. The category information you retrieve through the GetCategoryFeatures call includes metadata about the features that each category supports.

An application can also retrieve a list of items that other sellers offer within a specific category. This functionality is analogous to the browse screens of listings for categories seen on the eBay marketplace.

For more information about how sellers can select appropriate categories for their listings, refer to Adding a category to your listing.