if the blade was defective, then it isn't fair to "blame" the store. they don't get them for free, either.
if the blade was switched by a "customer", that is also not the store's fault. thieves are everywhere - including Rodeo Drive.
if you've checked every other possibility and the blade still doesn't perform to your needs or expectations, they will take it back. they will either give you a refund or an exchange.