First and foremost, run a carfax on the vehicle. If that turns up clean, then have the car taken to a reputable body shop and have them check to see if any paint-/body- work has been done to the car. A good body shop mechanic should immediately be able to tell you if the car has been in any sort of accident. Next, you should thoroughly review any paper work the guy has on the car (i.e. receipts, repairs, etc.). Then have the car taken to either an MB dealership or a reputable Mercedes mechanic and have them do the standard pre-purchase inspection. Make sure that they check the transmission thoroughly. Unfortunately the transmission is the Achilles heel of the C43 and is prone to fail at some point.
If all this turns up well and you feel like the seller is an honest person and you really like the car, start negotiating.
I should also note, to get an idea of how much a car is worth do not completely rely on KBB.com or Nada Guides. Log on to autotrader.com, cars.com and check out e-bay auctions to get a better understanding of what the market for these cars is. Good luck.