One of the most valuable of all classic Mercedes-Benz cars are the 300 SL Gullwing cars of the mid to late '50s. RM Sotheby's is set to auction off one of these rare cars and this one is even more special and rare than the norm. The car is a 1955 Mercedes-Benz 300 SL "Sportabteilung" Gullwing. The car is a …