This field is used to set the revised quantity of the listing (or of a variation within a multiple-variation listing).
In each InventoryStatus container, either StartPrice or Quantity (or both) are required.
The Quantity field is always returned in the response, regardless of whether the quantity was changed or not through a Quantity field in the call request. However, the Quantity field in the response is actually a total of the quantity available for sale plus the quantity already sold for the item or item variation. For example, suppose the item or item variation originally had a quantity of 10, and then a quantity of 8 was sold. Now, you restock your inventory, and you pass in a quantity of 10 in the ReviseInventoryStatus request. In this case, the response of this ReviseInventoryStatus call would show a quantity of 18 (10 available + 8 sold). To determine the quantity available, use the GetItem or GetSellerList call, and subtract the SellingStatus.QuantitySold value from the Quantity value. Or, you can also use the GetMyeBaySelling call, search for the correct item or item variation in the response by ItemID or SKU value, and then look at the QuantityAvailable field for that item or item variation.
It is a good idea to maintain an adequate quantity available for fixed-price GTC listings to prevent the search rankings from dropping. Best Match search ranking is based on buyer activity, and one of the factors affecting search ranking for fixed-price listings is the recent sales score. Fixed-price items that are selling the fastest are given a relative lift in search results.