Have a look here http://www.motortrend.com/cars/1999/mercedes_benz/c_class/specifications Then scroll down to the valuation section.

According to the chart, once a 99 C230 hits 200 K it has a negative value to a dealer but is over $ 3K as a consumer resale value.

Also, the consumer resale value price difference between 120 K and 210 K miles is ~ $ 1,600. I'd find paying that little for a car nearly 1/2 the miles is quite a bargain.