"I would say the buyer has taken leave of his senses. Colnago Supers are plentiful and cheap. Maybe there is something special about this particular one, but it isn't apparent to me."
I'd say that price was about right, or a little under-market, even.
Just try finding a 1973 Colnago like this, in your size, with clean original paint, those graphics, and the original, modified parts.
Most of all, just try finding one with the chromed head-lugs. Rare as hen's-teeth, lemme tell ya. I look for these constantly, and the ones with those graphics are getting harder and harder to find in any condition at all, period... let alone in clean, used condition. If the chrome was a little pitted, that's no big deal... the early Supers with chromed lugs are extremely rare, in my experience.
They're a bit easier to find in the later vintages...even then, though, they're not common.
So, the bidders for this bike knew exactly what they were bidding on, and no-one took leave of their senses. Quite the opposite. Someone got a really nice bike for a fair price.