What's up with the price of Vintage equipment?

The reason I am asking is, I sold stereo equipment in the 1970's and 1980's and some of the stuff I see selling for big bucks today were not well thought of during the 70's and 80's and lots of it sounded pretty mediocre (think Class D amps with amazingly low specs like .00007% distortion or other such nonsense). I won't get into brand names, but just by the era you should be able to tell. 

As a quick aside, it annoys the hell out of me that I'm at an age that 1970's and 1980's equipment is considered Vintage. Was it really that long ago? Rats.

Anyway, again there were some really good sounding stuff back then, and the price today may be high, but it's still good sounding stuff and worth it. But I see prices today for 40-60 year old equipment that I sold back then because it was heavily advertised, popular/known name and design (and sometimes gave me a good commission), but not because it sounded good. 

So I am assuming some of this high price vintage stuff is due only to nostalgia (both in looks and sound) and collecting fun, not for listening compared to today's stuff.

I'm open to hearing other opinions, especially from someone who does like vintage (mostly 1970-80's) stuff, listens to it, and what brands they have.