The MBG says the buyer has to provide "compelling evidence" that the item is counterfeit before they are not required to return the item for a refund, and they are also supposed to *cough* agree to destroy /  dispose of the item if they aren't required to return. However, the lack of "compelling evidence" won't mean that the claim will be decided in the seller's favour, it just means eBay don't let them keep it, in a nutshell. This kind of claim, without "compelling evidence" from the buyer, or irrefutable evidence to the contrary by the seller, will almost certainly result in the buyer being required to return for a full refund, at the seller's expense (I can't guarantee it, as these things sometimes have a tendency to be surprising in the worst ways 😞 ). . 

You can report the buyer for abuse of the MBG system regardless of how the case goes, it may not do anything right now, but if the buyer has or develops a habit of things like this, sooner or later eBay will take action (I mean seriously, counterfeit TS? I used to sell quite a lot of that, they're popular [or at least used to be] and aren't exactly cheap, but no one's falling over themselves to produce knockoffs),