You don't say whether the key blades, holders or both are bad. The holders have some sort of internal sensor that must be within several inches of the associated proper key to allow it to work. The proper key blade by itself won't start the car. My dealership had to send my keys back to Germany for programming when I had a new ignition put in. They said that there is no way to get this done locally: apparently this is a MB security feature.
If you buy a spare key and holder on ebay, you will have a programming headache! Notice that none of the sellers ever guarantees that these keys will work in your car.