The time to contact eBay to tell them that your buyer had opened an item not as described return when they hadn't yet collected the phone has passed. 

If a buyer opens a return you must accept it before they can ask eBay to step in (after 3 business days) which is now what seems to have happened here.

You can try appealing eBay's decision here:

https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369&from...

You have the IMEI number so report that phone as stolen.