Lot ID is the unique ID of an item in a seller's inventory generated at the time an item is added to the seller's inventory. It cannot be changed or reused. It is located below the image of each item in the seller's My Inventory detail pages. It is also referred to as inv ID.
You can find out the lot ID of an item for sale by doing a mouseover over the image of the item for sale (placing your mouse cursor over the image).