As covered already, not too much of anything here as a big tool to negotiate on the price. It's a 6 year old car and odds are they will use that to downplay the scuffs. Battery is something I'd definitely get them to cover getting done at the dealership as well as the maintenance. I've done a few battery's on newer MB's and some of them have random shit happen when you unhook the battery. Had two not want to start and another decide to just crank. All 3 resolved after a quick google search, but still scary on a high dollar (as far as a maintenance standpoint goes) car goes. Maintenance is a good thing is use to get te price down. That isn't something the car can or should go without as I'm sure you know. I'd really hammer at least getting the maintenance cost and battery cost off the price, or at least taken care of with proof, as well as a good detail while it's in there.
Good luck!