Abstract: A third party item listing management system useable for validation of third party items to be included on a retailer website is disclosed. The third party item listing management system includes an application programming interface (API) accessible by a plurality of third parties and configured to receive item data. An item management process receives the item data and calls an item validation pipeline which includes a plurality of item validation stages including an item legalization stage. In the item legalization stage, the item data and the identity of the third party are validated against a plurality of item listing rules to determine whether the one or more items are allowed to be offered via the retailer website by the third party. The item listing rules can include a rule preventing the third party from listing an item included in a core item collection offered by the retailer via the retailer website.