When listing an item, the seller can optionally designate the listing as private. Doing so hides the user IDs of bidders from users viewing the active listing, except for the seller. This provides anonymity to the listing's bidders.

A seller can designate auction and fixed-price listings as private. Sellers are advised to only use this designation when they have a specific reason to do so.

On the eBay marketplace, the seller enables this option when defining the new item on the Sell Your Item (SYI) page. In the eBay API, this is done by passing a value of true for the Item.PrivateListing property when using AddItem. The Private argument is also available for use with RelistItem and ReviseItem.

When an active listing is viewed on the eBay marketplace on the View Item (VI) page by anyone but the seller, the list of bidders is not displayed. Even for the seller, visible bidder user IDs are limited to the high bidder while the listing is still active. The seller is, of necessity, able to see the winning bidder after a competitive-bid item listing (an auction) has ended. In the API, the private designation is indicated by a value of true returned in the Item.PrivateListing when items are retrieved with such calls as GetSellerList and GetItem. For calls that return bidder user IDs (such as in the Item.HighBidder node of the GetItem result set, the high bidder user ID is only visible if the function requestor is the listing's seller.

When a user ID is hidden in an API call result set due to the private auction designation, the user ID field is still returned. But the value the field contains is "Invalid Request" (without the quotation marks) instead of the actual user ID.

For additional information about creating private listings, refer to Private listings.