6. A non-transitory computer-readable storage medium having computer-executable instructions stored thereon that, when executed by a computer, cause the computer to: maintain feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer-facing data for listing of items in an item catalog and a second set of semantics defined for a feature of the item catalog, the second set of semantics describing a transformation of an attribute related to the feature of the item catalog; receive supplemental feed data from a seller comprising a value for the attribute related to the feature for an item listed in the item catalog, the supplemental feed data further indicating a target feature of the item catalog; validate the value for the attribute in the supplemental feed data based on the second set of semantics defined for the target feature of the item catalog; and transform the supplemental feed data to supplemental listings data for inclusion in the item catalog based on the second set of semantics defined for the target feature of the item catalog, wherein the value for the attribute of the item in the supplemental listings data is merged with listings data containing values for attributes for the item periodically loaded from seller feed data based on the first set of semantics in the feed metadata.
6. A non-transitory computer-readable storage medium having computer-executable instructions stored thereon that, when executed by a computer, cause the computer to: maintain feed metadata comprising a first set of semantics describing a transformation of seller feed data to customer-facing data for listing of items in an item catalog and a second set of semantics defined for a feature of the item catalog, the second set of semantics describing a transformation of an attribute related to the feature of the item catalog; receive supplemental feed data from a seller comprising a value for the attribute related to the feature for an item listed in the item catalog, the supplemental feed data further indicating a target feature of the item catalog; validate the value for the attribute in the supplemental feed data based on the second set of semantics defined for the target feature of the item catalog; and transform the supplemental feed data to supplemental listings data for inclusion in the item catalog based on the second set of semantics defined for the target feature of the item catalog, wherein the value for the attribute of the item in the supplemental listings data is merged with listings data containing values for attributes for the item periodically loaded from seller feed data based on the first set of semantics in the feed metadata. 7. The computer-readable storage medium of claim 6 , 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.