So your buyer re listed and sold it on?
What do you do? nothing UNLESS, your buyer has re-listed it, sold it and had it returned for a refund in less than the 45 days from his payment date to you.
If the above is correct, then he could open a dispute with eBay. They will not ask him to prove it is fake, but they may ask you to prove it is not
If this were me, I would not accept a return if it was outside the 45 day time period let your buyer sort it out with his buyer.
Never, ever refund without the return of the item, ever
And no, you need not do anything with regard to contacting eBay. At the moment this item has nothing to do with you