The bidder/buyer was already told the item looks fine.Per Ebay rules, you have to pay for this item now that you have won it unless the seller will agree to cancel the sale. It is considered a legal binding contract. However, if you decide not to pay the only thing they can do to you is give your account a violation, so you may not be able to purchase items in the future. You should pay and when you receive the item take a bunch of pictures and submit them here for authentication. If it is not authentic, the seller has to accept a return and ebay will refund your money except the shipping charge.
Addition: I believe I just found the item you won: ebay item # 270729837919
I have seen this bag at the Coach factory store. Since I am not an expert I can't authenticate it, but I wouldn't worry.
But to respond to your comment, although a buyer should authenticate items prior to bidding, she certainly isn't obligated (contract or not) to pay for a fake.
It's a PITA to fight with a seller about SNAD after having paid for the item plus shipping. If and when a buyer learns BEFORE paying that she bought a fake, she shouldn't go through with the purchase. "per ebay rules," the seller shouldn't have listed a fake and "per ebay rules," the buyer isn't required to pay for a counterfeit item.