ShipStation now receives an item's Marketplace SKU from Etsy when it is available on a product, instead of receiving an item's Listing ID from Etsy.
To learn more about Etsy's Marketplace SKUs, read their article Using SKU for Your Inventory
You can create SKUs yourself within Etsy, and once the orders import into ShipStation, the product record will add a more control and customization over your Etsy orders' fulfillment!
Review the article Why are some of my products missing in ShipStation? to learn more about SKUs and troubleshooting missing product imports within ShipStation!