You can contact them and make them aware of the situation although they will probably leave it for the Police to investigate. Assuming you have a CRN, then ask them to make a note of it on your account and the sellers account.
This really is in the hands of the Police and all you can do is provide them with as much information as you can and let them investigate the crime.
https://www.ebay.co.uk/help/eua?id=5190
Edit: meant to add, take as many screenshots as you can of the listings and an estimated stock value. Items for sale and sold. They may delete the active listings and Ebay may not provide this information in the future so collect as much evidence as you can.