In the context of eBay traditional APIs, a user is someone who has registered as a member of eBay. Anyone can browse the eBay site without registering as a user. However, eBay traditional APIs functionality support only registered users.
An eBay user has a unique identifier by which the user can be referred. This user ID is unique across all eBay marketplaces.
In the APIs, a user is represented by the User object. This object contains general information about a user, such as the user ID, whether the user is a new eBay member, and the eBay site on which the user registered. It also contains information that plays a part in buying or selling an item such as the user's aggregate Feedback score, whether the user is an eBay Store owner and, if so, the Store's location, and whether the user is a Power Seller. The data for a user can be retrieved by calling GetUser which populates a User object with data it retrieves from eBay.
Users who participate in activities on eBay, whether through the site or an application, do so in a variety of roles. eBay roles are not mutually exclusive so the same user may be a seller in one order and a buyer in another order. An eBay membership is generic in terms of user roles so there is no membership type that is specific to sellers and another to buyers. A user simply acts in one role or capacity as they need or want to do all using the same user identity.
Registered eBay users
Anyone may visit an eBay site, browse items offered for sale, peruse the available jobs list, and read the online help for the site. But to do anything meaningful — such as selling and/or buying items — a user must become a registered eBay user. Becoming an eBay member is free and registration is easy. Registering supplies eBay with a means to contact the user and results in the user having a unique user ID.
Interactions on eBay are relatively anonymous. While searching for and browsing listings, a user will only see the seller's and other buyers'/bidders' user IDs, and not their real names. Interactions on eBay are not, however, completely anonymous. eBay does have a real name and contact method for each user, and the seller will see their order partner's (buyer's) real name if that name is a part of the shipping address.
Application developers
An application developer is someone who either develops API applications or acts as the representative of a company that develops such applications. In the case of the latter, the company may have hired one or more developers to write the program's code, but the user referred to as "application developer" in this context is the one who interacts with the eBay Developers Program.
To develop applications that interact with eBay through the APIs and/or SDKs, a user must register an account with the eBay Developer's Program. After registering, they will use these account credentials to log into the developer portal, and their account ID will be associated with the AppID that is needed to run eBay APIs.
As a developer, you will want to test your APIs using the eBay Sandbox environment. To do this, you will need to create several Sandbox user accounts, with each account representing different, fictitious eBay buyers and sellers. These account IDs will only be valid for the Sandbox, and cannot be used on the production eBay site.
Sellers
A seller is an eBay user selling an item on eBay. To sell an item on eBay, a person must be a registered eBay user, and must also set up a selling account.
Store Owners
A store owner is a registered eBay user who has an eBay Stores subscription. With an eBay Stores subscription, a seller gets an eBay Stores storefront. For additional information, refer to Open and manage an eBay store.
Bidders and buyers
Bidders and buyers are the users who are either buying an item or who are attempting to do so by becoming the winning bidder for an auction item.
A bidder is a user who makes a bid on an item in an auction listing. Competing with other bidders in a series of progressively higher bids, a bidder vies to be the highest bidder when the listing ends. A bidder who is the high bidder (and also meets the Reserve Price, if set) when an auction listing ends is deemed the winning bidder and is able to purchase the item from the seller.
A buyer is a user who has or is able to purchase an item from the seller. In auction listings, a user is only able to purchase the item after the listing has ended and the user is the winning bidder for the item. For fixed-price listings (and auction listings with an active Buy It Now option), a user simply purchases the item at the specified fixed price.