It now appears that the seller did get the violin authenticated.

From a brief search, it looks like eBay had to pay large sums multiple times for selling counterfeit goods. $2500 violin, as sad as it is, is small fries. It seems that it's cheaper to operate under the assumption that if there's a suspicion an item is counterfeit, then that's more likely than not. Even if they messed up and lose a lawsuit from a seller, they'll pay a small fraction of what they'd pay a manufacturer.