I made a call to GetSellerList with a single SKU, but the response returns no results. If I call GetItem with the ItemID I get the Item, along with the correct SKU.
Answer
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.