I've used them and been extremely pleased on the buy side. Thought I could do better on the sell side privately though. As the above poster says, check it all the time, it's amazing how quickly the good stuff goes. I got what I consider to be an absolute steal on a mk1 200mm f2.8, the one with the built-in hood. Same optics as the mk2, just more convenient. It was 18 years old and looks like it just came out of the factory. Amazing.