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<LongMessage>The quantity you submitted is invalid. The quantity should be greater than 0 for an active item.</LongMessage>
As indicated by the error message quantity field sent in should be > 0. If you need to pull down an item from eBay use the EndFixedPriceItem call.
2. I am getting a "Invalid SKU number: 00135550302010" error when I make a call to ReviseInventoryStatus. I know I
sent in the SKU number when I listed the item. Why am I getting the error?
SKU can only be used to revise 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; or
In the ReviseInventoryStatus request, if the listing has InventoryTrackingMethod set to SKU, then 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.