Part of it is taking good car of it. My really old stuff has more paint loss than would be allowable for EX, but we're talking equipment that I have had for 15+ years and used daily. 15 years is really old to me because I am only 32 years old. Still, none of my stuff is beat up..no dents, no flaws or scratches on the lens glass, etc. It all works well because I have taken good care of it (kept it in bags, or in my cabinets when not in use), and had the things serviced periodically.
I think a lot of the stuff KEH sells that are in really nice shape were just not used much. Many amateurs have a lot of money and buy things like Hasselblads or Nikon F5's or Leicas and rarely use them to take actual photos....they just have them to brag about being "Photographers". These are great people to buy gear from cause its nearly new! Those of us who do this professionally and use the gear daily cannot keep it in flawless shape no matter how well we treat it.