As the buyer is in the US, eBay's Money Back Guarantee will be in effect as that's been available to US buyers for quite some time.

If the buyer's messages can show that the real reason for return is change of mind, you should be able to win the case (not guaranteed, of course), if their messages consistently indicate (even vaguely) that the issue is to do with the phone case and not whether they like it etc, I suspect your chances would be very slim (keep in mind that if you lose an escalated case, it becomes a case closed without seller resolution, of which you're allowed 2 or 0.3% in any given evaluation period before you're relegated to "below standard" and account sanctions will likely apply). 

If the request is escalated to a case, I honestly don't know at this stage what eBay require from the buyer - it might be worthwhile asking on the US forums, as sellers there would likely have a lot more experience dealing with eBay's MBG cases.