We have noticed a number of Mercedes-Benz keys being sold here on ebay, they are black (latest have chrome on each side) with either 2 or 3 buttons and are known as DAS3 by Mercedes-Benz. 

The two on the left of the picture.

 To clarify these keys cannot be re-coded in order for them to work on a different vehicle, not even by a Mercedes-Benz dealer. This is a vehicle security feature, so to buy a used one is a not a good idea.

Infra-red Flip keys (The metal blade flips out far right in picture) which are used on certain C class MLs SLKs and A class amongst others, cannot be re-coded in order for them to work on a different vehicle. However, if this type of key isn't used for an extended period, or if the key batteries go flat  they too "forget" (a vehicle security feature) the unlocking code for the vehicle they were coded to. These keys can be taught to re-learn the orginal code as long as the key isnt faulty.

This procedure can be found in the relevant owners manual and takes just moments to complete or just pop in and see us and we can do it for you.

Its best with all Mercedes-Benz remote locking keys to rotate there use every so often, and if you havent got a spare we strongly suggest you invest in one as it saves time and money if you loose the only key you have.

Hope this helps!

Mercport