There was an article somewhere about some Mercedes Dealerships who were "up selling" an AMG Upgrade for thousands, and just adding a Badge and a set of Wheels.
These days these Cars are now called AMG "line", I know that because my 2017 E Class ex demonstrator was such an animal, came from the factory that way !!
But yes, there are plenty posers who add badges, even some like myself who add Brabus badges and other bits and pieces to their (currently) V6 R170
It is however great fun to be sat outside a Pub or Cafe, next to un initiated "Sports Car experts" who have scrutinised your Car and are having a discussion as to why your (atm) almost stock SLK 320 is a "Genuine Brabus" because you can tell by the Stainless Rollbars, Paddle Shifts etc etc blah blah
I was itching to join that conversation, but Mel wouldn't let me
Personally I don't have a problem with it as long as a seller is not trying to pass a Car off as such for extra money, and provided of course the Car in question is upgraded and not just a standard one, there are "Hot Rodders" all around the world