THIS IS NOT ACCURATE. I am a former electronics engineer and operates a car diagnostics business.A regularly repair Mercedes electronic ignition switch eis, electronic steering locks ESL and i have done countless of these keys.
I am not poaching for business just want to set the record straight.
While I agree that very very few people have the equipment or the knowledge to programme them it can be done.
I do them on a daily basis. As long as the key you buy is fully working and your original key is working it can be done with the right equipment.
If you buy a key off eBay or from China and walk into a Mercedes dealer asking them to programme it of course they are going to chase you away. After all they make £300 a time for selling you a key which is equivalent to millions of Pounds per year. Also the dealers do not make keys they send to Germany for them which takes about 5 days.
Also contrary to popular opinion by reading certain processors on the car Mercedes key dumps can be calculated and used to make a key to drive the vehicle, again knowlledge, skill and equipment is required. Remember Its just electronics not magic.
The advice is
1.Make sure you buy a good working key preferably the Chrome one.Not the old plastic ones(or from cars around 2002 upwards)
2. Find the right person who has experience in doing it.
3. Bring along your car and working key the process can be done in no time.