Definitely have it authenticated. 100% feedback means nothing. I would go ahead and open a return case to buy some time. Maybe someone else can give you the name of a good authenticator. You need to hurry before the seller gets there money. If you paid per PayPal you can still open a case but it's more complicated. Your easiest, quickest route for return is through EBay. Good luck.
Unfortunately YSL is a brand I'm not too familiar with. Maybe the authenticators on the YSL forum or someone in YSL chat thread can give you some guidance. I hope it's not too late for you to open an eBay Not As Described case. You need to go ahead an open a case.
Good luck.