15. A method for supplementing customer-facing listings in an item catalog, the method comprising: receiving, at one or more computers, a selection of a feature of the item catalog; generating, by the one or more computers, a supplemental feed template from a second set of semantics maintained in feed metadata, the second set of semantics corresponding to an attribute of items in the item catalog, the attribute related to the selected feature of the item catalog, the supplemental feed template comprising header information identifying the selected feature of the item catalog; receiving, at the one or more computers, supplemental feed data from a seller comprising a value for the attribute of an item listed in the item catalog, the supplemental feed data created from the supplemental feed template; validating, by the one or more computers, the supplemental feed data based on the second set of semantics corresponding to the attribute, wherein the feed metadata further comprises a first set of semantics describing a transformation of seller feed data to customer-facing data for inclusion in the item catalog; transforming, by the one or more computers, the supplemental feed data to supplemental listings data for the item based on the second set of semantics corresponding to the; and merging, by the one or more computers, the supplemental listings data into the item catalog containing customer-facing listings data corresponding to the item loaded from seller feed data based on the first set of semantics, wherein the value for the attribute in the supplemental listings data takes precedence over a value for the attribute in the item catalog loaded from the seller feed data.
15. A method for supplementing customer-facing listings in an item catalog, the method comprising: receiving, at one or more computers, a selection of a feature of the item catalog; generating, by the one or more computers, a supplemental feed template from a second set of semantics maintained in feed metadata, the second set of semantics corresponding to an attribute of items in the item catalog, the attribute related to the selected feature of the item catalog, the supplemental feed template comprising header information identifying the selected feature of the item catalog; receiving, at the one or more computers, supplemental feed data from a seller comprising a value for the attribute of an item listed in the item catalog, the supplemental feed data created from the supplemental feed template; validating, by the one or more computers, the supplemental feed data based on the second set of semantics corresponding to the attribute, wherein the feed metadata further comprises a first set of semantics describing a transformation of seller feed data to customer-facing data for inclusion in the item catalog; transforming, by the one or more computers, the supplemental feed data to supplemental listings data for the item based on the second set of semantics corresponding to the; and merging, by the one or more computers, the supplemental listings data into the item catalog containing customer-facing listings data corresponding to the item loaded from seller feed data based on the first set of semantics, wherein the value for the attribute in the supplemental listings data takes precedence over a value for the attribute in the item catalog loaded from the seller feed data. 21. The method of claim 15 , wherein the supplemental feed data comprises an expression to be used to compute the value for the attribute of the item in the supplemental listings data.