Your watch was listed with ebay Simple Delivery which would have covered the value up to £750 and would have used a tracked service.
As you have used your own postage, you can't claim the payment back - the buyer paid Ebay for Simple Delivery so the buyer will be refunded.
You can't change the tracking but it is a good idea to contact the buyer to give them the correct tracking number.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)