I am sure that someone else can be more precise on this, but Shimano 600 components would be minimally 5-10 years old. 600 was good stuff, but always below Dura Ace, Shimano's top of the line.

I am going to hazard a guess that most of the stuff on the bike is older than the gent is letting on--the components are probably original, which means mid-1980s. But, that doesn't mean that the bike is overpriced. If the frame is still in good shape, it may soon start to have collector's value.

If you are buying it for riding (and not just collecting), make sure it fits. If it doesn't fit, then any amount of money would make it overpriced.

Cheers,
Jamie