Buyer want to return my New GoPro after 2 weeks use, T.T

I sold my brand new GoPro on eBay 2 weeks ago, nothing happened in the last 2 weeks, however, after 2 weeks, today, I received a return request from buyer, he opened a "Doesn't work or is defective" case against me and describe that he found the water go through the screen and he lost his footage. And what he want is FULL REFUND. I was totally shocked, since its a nearly 200 quids deal.
My point is what I sold is SEALED and New item.
And he already USED 2 weeks. and full refund is unacceptable.
This is my 5th time sell on eBay. I'm so upset right now, and I heard eBay always on buyer's side. : (
What can I do right now? Appreciate for any comments and suggestions.

Source: https://www.hotukdeals.com/discussions/buyer-want-to-return-my-new-gopro-after-2-weeks-use-tt-2975371