The unique SKU of the item being ended. A SKU (stock keeping unit) is an identifier defined by a seller. SKU can only be used to end an item if you listed the item by using AddFixedPriceItem or RelistFixedPriceItem, and you set Item.InventoryTrackingMethod to SKU at the time the item was listed. (These criteria are necessary to uniquely identify the listing by a SKU.)
In the EndFixedPriceItem request, either ItemID or SKU is required. If both are passed in and they don't refer to the same listing, eBay ignores SKU and considers only the ItemID.
To remove a SKU when you revise or relist an item, use DeletedField in the revision or relist call.