SKU can be used as an identifier for a listing only if the item was listed with InventoryTrackingMethod as SKU. In your case it appears that the item was not listed with InventoryTrackingMethod as SKU and therefore cannot be pulled by passing the SKU in the GetSellerList request.
How well did this answer your question?
Thank you for helping us to improve the eBay developer program.
While we are not able to respond directly to your comments, we will carefully review them to improve your experience with eBay developer program!