i reckon the pre 80 market has eased right off because of the age of the demographic. most blokes in their 60's can't be stuffed getting on the old dunga's from the 70's anymore.
pre 90 stuff is where it's at right now. the average buyer is in their 40's to 50's and is cashed up and still fit enough to ride.
the big money is in anything 500cc and 2 stroke. wish i had a shed full of CR500's