The Cross-Sell Box is a meta box that automatically pulls other products or items you are selling on your eBay seller account or eBay store. This will help you promote and encourage your buyer to check out your other listings.
Unfortunately, since eBay no longer allows active content, it is not able to dynamically fetch the listings based on "keywords" so it will automatically load the related products based on "eBay's best match".
To add a Cross-Sell box, click the "Elements" tab and find the " Cross-Sell Box" widget from the list. Drag and drop the widget on the spot where you wish to place the box. You can click the edit button to change the icon image and header title. The items will automatically populate in the box once you apply the template and revise your listing using the revision tool.