Abstract: Disclosed are various embodiments for custom result set definitions. A result set definition is obtained responsive to a request from a client. A result customization application generates and executes application program interface calls to generate response data having a fixed schema. The result customization application performs transformations on the response data to generate a result set conforming to the result set definition.

Abstract: A merchant system is disclosed that can provide multiple online marketplaces that utilize marketplace product catalog layering. Product catalogs associated with each marketplace may be defined and layered such that products sold by one marketplace may be displayed on and purchased by way of another marketplace. The merchant system might also provide a unified electronic shopping cart for storing purchases made at multiple marketplaces, a unified checkout user interface for completing the purchase of items from multiple marketplaces, and a unified order management user interface for managing purchases made from multiple marketplaces.