this page introduced requirements for product identifiers eBay's Spring 2015 Update eBay Product Identifiers MPN identifier eBay's Product Identifier (GTIN) requirement introduced requirements for product identifiers eBay's Product Identifier (GTIN) requirement product identifiers "Product Identifiers | eBay Seller Center" the new requirement eBay's Spring 2015 Update initiative Product Identifier Frequent Questions old eBay help page