Any key that has already been programmed to a Mercdes will never work in any other car.
Once a key has been "flashed" or programmed to one car you can't reprogram the key. Mercedes keys pair "for life", so once you stick a key in the ignition and it works, it's never going to work in another car.
It is true that a Mercedes can accept new keys; when you get a new key you usually get an "additional" key, which pairs with the car and programs very quickly. You can have multiple keys that work with a car, but you'll have to get them ordered from Mercedes; dealers can't make them up and they won't have them on hand. You'll have to place an order with your VIN number for them to send you a compatible, fresh key.
There have been claims that you can reprogram a smart key but this is not true once the key has been paired. If you order online and get a used, preprogrammed Mercedes key, it's not going to work. It might not even work with the car it was originally programmed for!