Related Store View serves as a source of the Magento product data for item mapping. To synchronize the product data correctly, you need to associate the 3rd Party Items imported from the particular eBay site with the related Magento Store Views.
To set up Store Views with eBay sites, navigate to Configuration > Accounts > Unmanaged listings > Related Store Views. There you’ll find the list of eBay sites and will be able to choose the right Store View from the drop-down for each site.
If there is no needed eBay site in this list, please navigate to eBay > Configuration > eBay Sites to enable it.