Jet.com is a unique marketplace in that you cannot work only within jet.com's seller portal. You will need to use a third party software to communicate order and product information.
For customers currently, or considering, using ShipStation, you can integrate fully for all order, product, stock, and returns management within ShipStation. The inventory control can be set up one of two ways:
1) Finale can communicate directly to ShipStation, and ShipStation will then communicate to the marketplace. This will allow lsting management and creation within ShipStation.
2) Finale can be configured to communicate directly to Jet.com. Please visit Configuring Finale to push the stock levels to Amazon, eBay, WooCommerce, Shopify and other selling channels to configure this. One thing to note, if you connect directly to Jet.com, you will need to deactivate the product and stock management within ShipStation first. This will also disable the ability to create and manage your listings. This is not advised. How to deactivate ShipStation's Inventory Management (external link)
If you're leveraging the third party software Geekseller, please visit How to push stock counts to Jet.com if you're using Geekseller to set up the connection.