One of the ideas I always try to stress is that, while assymetric information exists in exchanges, it still takes supply AND demand to make a market.
My favorite illustration for markets without demand was an old Calvin & Hobbes comic. Calvin had set up a booth and was selling "swift kick in the butt". Business was terrible, and he told Hobbes he couldn't understand it. "Everyone I know needs what I'm selling." A clear case of supply but no demand. ***UPDATE*** (You can find it here. HT to Dr. Mark Witte at Northwestern University for the link.)
On the other side, I can think of no better example of demand without supply than this classic bit from Monty Python's Flying Circus. Enjoy your weekend.