After a little bit of research, it seems that after the collapse of the Soviet Union in 1993 the coins were taken out of circulation and were no longer accepted as currency within Russia. If they are as rare as the seller suggests, then the face value of the coin is irrelevant as you'd never trade it in on that basis anyway.
Exactly. The Soviet Union no longer exists - would be fun in today's world if it would... The modern Russian ruble is a different coin, featuring the modern state emblem of Russia (the double headed eagle) and it is smaller (because of inflation).
The coins offered here are collectible items.
The two years/variants you're mentioning are ultra rare and therefore extremely expensive. But the year 1978 is still more rare than other years. In general, you will not often find the coin in a comparably excellent condition as offered here.
I think the coins are a great memorial set of an important and fascinating part of history.