Oh, I remember this thing...
The guy who was selling it originally (maybe the same guy now, I don't know) used to put a long backstory on why it was so rare....basically, he said that in Japan there was a limited edition of Darius that was being sold in certain game stores in very limited quantities. Then, if you were lucky enough to get a copy, you'd be entered into a raffle to win a super-rare, never-to-be-sold-in-stores game: Darius Alpha (or whatever it's called).
The game in the auction was ostensibly won in the ensuing raffle...that's why it says "Not for sale".
Of course, the story could be completely bogus.