The keys are coded to the VIN #, all the remotes on E-Bay (smart-keys) are useless except for repacing the outside case or to impress people on your key chain.
When the car is mfd. the remotes are #1, #2 and the emergency electronic (very slim with no buttons but shape of remote)key#3. The dealer can read your key and see which one you have, and then either order the replacement or an additional key. The replacement will disable the previous key, the additional will add a key (up to eight) after that some items will have to be replaced and you can start all over with new remotes and engine control module. There is a maximum of actions 24 per vehicle. A replacement key will unlock the doors but will not turn for up to 90 minutes after inserting in lock, it will then turn and possibly go another 90 minutes before the car will start and keep running, while it "communicates" with the computer. The MB parts depot keeps a record of the key assignments and programs the key according to the VIN and the request is filled usually within a couple of days. The key also draws its power from the car so if the batteries are dead you can start the car.
Remotes can be deactivated by the dealer but never "coded or recoded", as a matter of fact if the system is replaced it is shipped with a "workshop" key that transferes data to the computer and then each new remote must be inserted while data is transferred before anything works. My warning about E- Bay stands!