The idea here is that if a city is offering something for sale at a
particular price and isn't selling any, the price will get marked
down. If it is offering something for sale and is selling it, the
price will get marked up. Buying works just the opposite.
This would have several good effects, the most important being that
trading would be more viable.
I might also suggest that cities should not sell goods that can be
COLLECTed by players, but that's another issue.
-- Scott T.