Unfortunately, the ECM is vehicle specific. The VIN has to be programmed in at the parts location. They have the software, the dealer doesn't. Once it gets to the dealer, the rest of the vehicle info gets programmed in. To my knowledge, Mercedes has not released the software so ECM's can be recycled.
I'll give you another. To buy a Sprinter key, you have to order it from the dealer, who orders it from the warehouse. Upon receipt of the key, the dealer will cut it based on the VIN. The dealer then sends the key to a company in CA to have the proper crystal installed and they send it back to the dealer. When you pick up your key in about 10 days or so and hand the dealer $300, you're good to go.
All Mercedes vehicles are set up this way should you ever decide to buy one.