Dealer margins are tight, so they won't replace tyres unless absolutely necessary, as mentioned earlier. They will sell the car with the original tires as long as the tread is within the MB approved used acceptable limit (about 3mm), which is based on UK standards. I don't think cheap tires necessarily mean the car hasn't been well-maintained; after all, not everyone has the same level of knowledge about tires as, instance, a car enthusiast. While I would prefer to see name-brand tires on a used car, I wouldn't let that deter me from buying the vehicle otherwise—I would just make sure to include the cost of replacing the tires in my budget.