22. A metadata-driven supplemental feed processing system comprising: one or more application servers; a metadata service providing feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer facing data for inclusion in an item catalog; a template generator module executing on the one or more application servers, operatively connected to the metadata service, and configured to receive a selection of a feature of the item catalog, and generate a supplemental feed template file from a second set of semantics in the feed metadata, the second set of semantics describing a transformation of an attribute related to the selected feature of the item catalog; and a supplemental feed processing module executing on the one or more application servers, operatively connected to the metadata service, and configured to receive supplemental feed data from a seller comprising a value for the attribute related to the selected feature for of an item listing in the item catalog, wherein the supplemental feed data is created from the supplemental feed template file, validate the value for the attribute in the supplemental feed data based on the second set of semantics in the feed metadata, transform the supplemental feed data to supplemental listings data for the item for inclusion in the item catalog based on the second set of semantics in the feed metadata; and merge the value for the attribute in the supplemental listings data with listings data containing values for customer-facing attributes for the item periodically loaded from seller feed data based on the first set of semantics.
22. A metadata-driven supplemental feed processing system comprising: one or more application servers; a metadata service providing feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer facing data for inclusion in an item catalog; a template generator module executing on the one or more application servers, operatively connected to the metadata service, and configured to receive a selection of a feature of the item catalog, and generate a supplemental feed template file from a second set of semantics in the feed metadata, the second set of semantics describing a transformation of an attribute related to the selected feature of the item catalog; and a supplemental feed processing module executing on the one or more application servers, operatively connected to the metadata service, and configured to receive supplemental feed data from a seller comprising a value for the attribute related to the selected feature for of an item listing in the item catalog, wherein the supplemental feed data is created from the supplemental feed template file, validate the value for the attribute in the supplemental feed data based on the second set of semantics in the feed metadata, transform the supplemental feed data to supplemental listings data for the item for inclusion in the item catalog based on the second set of semantics in the feed metadata; and merge the value for the attribute in the supplemental listings data with listings data containing values for customer-facing attributes for the item periodically loaded from seller feed data based on the first set of semantics. 26. The system of claim 22 , wherein the supplemental feed template comprises a flat file having rows corresponding to one or more distinct items and columns corresponding to one or more seller-facing fields corresponding to the attribute related to the feature of the item catalog.