If you want to add past inventory into Sonlet, you can do this using a stock photo, or other photo, to load with your item. 

You can then mark those items individually as "Sold Outside of Sonlet." You will find the item on your Inventory page, and click the arrows button under Tools.

You can select a previous customer or enter new customer information. You can back date your previous Sales on the "Sold Outside of Sonlet" page.

If you want to mark more than one item at a time, you can move the items over to your Listings page. From there you can select all the items you want to mark, scroll down to click "Sold Selected Outside of Sonlet." 

You will be taken to the customer information page to complete the sale.