eBay syncing error: Error received updating Quantity to 4 for EBay Storefront: eBay. Error:Valid Listing type for fixedprice apis are FixedPriceItem and StoresFixedPrice. 

The error you are seeing on the sync inventory history screen means you may have an auction listing with the same custom label as a fixed price listing and the updates are sent to eBay based on quantity changes. After the updates are sent, eBay lets you know you cannot change the balance for an auction listing unless the balance is going from 1 to 0. If you have a fixed price listing tied to that custom label, you should see the balance updated successfully.