1. A method for supplementing customer-facing listings in an item catalog, the method comprising: maintaining in a merchant system feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer-facing data for inclusion in the item catalog; receiving a selection of a feature of the item catalog; generating, by one or more computers in the merchant system, a supplemental feed template from a second set of semantics in the feed metadata, the second set of semantics describing a transformation of one or more attributes related to the selected feature of the item catalog; receiving, at the one or more computers, supplemental feed data from a seller comprising values for the one or more attributes related to the selected feature for an existing item listed in the item catalog, wherein the supplemental feed data is created from the supplemental feed template; validating, by the one or more computers, the supplemental feed data based on the second set of semantics in the feed metadata; transforming, by the one or more computers, the supplemental feed data to supplemental listings data for the existing item based on the second set of semantics in the feed metadata; and merging the values for the one or more attributes related to the feature of the item catalog in the supplemental listings data with item details for the existing item in the item catalog, wherein the item details for the existing item comprise values for customer-facing attributes of the existing item periodically loaded from seller feed data based on the first set of semantics in the feed metadata.
1. A method for supplementing customer-facing listings in an item catalog, the method comprising: maintaining in a merchant system feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer-facing data for inclusion in the item catalog; receiving a selection of a feature of the item catalog; generating, by one or more computers in the merchant system, a supplemental feed template from a second set of semantics in the feed metadata, the second set of semantics describing a transformation of one or more attributes related to the selected feature of the item catalog; receiving, at the one or more computers, supplemental feed data from a seller comprising values for the one or more attributes related to the selected feature for an existing item listed in the item catalog, wherein the supplemental feed data is created from the supplemental feed template; validating, by the one or more computers, the supplemental feed data based on the second set of semantics in the feed metadata; transforming, by the one or more computers, the supplemental feed data to supplemental listings data for the existing item based on the second set of semantics in the feed metadata; and merging the values for the one or more attributes related to the feature of the item catalog in the supplemental listings data with item details for the existing item in the item catalog, wherein the item details for the existing item comprise values for customer-facing attributes of the existing item periodically loaded from seller feed data based on the first set of semantics in the feed metadata. 2. The method of claim 1 , wherein the value for one of the one or more attributes in the supplemental listings data takes precedence over a value for the attribute of the item in the item catalog loaded from the seller feed data.