Not really, the seller is in the wrong here not the buyer.
I have no interest in the military and wouldn't know the difference between that helmet and a mk1, so if I was in the market and liked the look of it I wouldn't have a problem buying it.
I'd like to think if your buying this kind of item you've got some idea of what it is your putting on your head and where it originates.
Don't get me wrong, the seller is 100% the issue here, but I'd hope that anyone worth their salt spending (I'd expect quite a considerable) amount on a helmet to be aware of it's route to market.
Unfortunately, there are still people rocking awful 6/6A's at formed Units, because there aren't enough of these (amongst other item's) in the system to do so, and it's definitely not helped by someone banging their kit on eBay at the end of their careers.