Provided the buyer uses Paypal, eBay will side with the buyer if he finds out that the watches are reps and he'll get a Paypal refund and the seller's Paypal account would be charged back. Also, the seller listed a 14 day return policy. So, even if the watches were genuine, the buyer could still return them. If the seller tried to renege on the return policy, eBay would still side with the buyer and issue a Paypal refund and a charge-back to the seller's account. The seller would also risk having their eBay account suspended.
Really, only two ways this turns out bad for the buyer. One, if the buyer doesn't take the watches to an AD to get them authenticated right away. If he doesn't know they are fake, he won't initiate a case with eBay. Two, the buyer doesn't use Paypal. Ebay owns Paypal. If the buyer pays with another method, eBay can't refund the money, even if they side with the buyer. All they could do is suspend the seller's eBay account.