The OP's car does have an MB warranty on it according to the ad.
Might not be a year but could be extended come renewal - it seems to now be way down on the initial asking price, which is about right all things considered and the car is well down on the cheapest pre-facelift, albeit 13 plate with less miles, on the MB AUC (£31k).
I've seen quite a lot of cars over the years listed on here starting with 2,3,4 so quite normal in my experience.
You get more provenance with an enthusiast-owned car than you do from a dealer who knows nothing about the history of their car. But then that's partly why you stump up more at an MB dealer - for the protection against the unknown.
Each to their own of course.