Yep, I would have bought it, too. The Mark IIIs were not known for being a great revolver, though by today's standards it would be a nice gun to have in one's collection. The Mark V was a better designed pistol and was more accurate, plus the grip was a great improvement over that of the Mark III. Why Colt put the grips it did on the Mark III and the Python I'll never know, but they was great if you had a hand like an orangutan!
Just the workmanship alone would be worth the three hundred bucks, and the bluing. A S&W 19 and the Ruger Security-Six were better guns, IMO, but the Mark III was a Colt, and for that reason alone I'd buy it.