This is definitely an issue in single player, though I don't know about multi player.
You can put up resources for more than market price, and the computer seems forced to always buy them first. I.e. Crystal starts out at 300/200. If you instantly put up crystal selling for 400, the computer will buy all of it eventually, even though the 'market price' is still only 300 credits. I hope this just the computer playing stupidly, and that other players aren't forced to buy a player's goods, even if it's more expensive than the 'neutral' shop, as the player could just keep buying goods for 300 and selling for 400, and the other player would never really be able to break his monopoly (well, in a 1v1 at least).
Edit: (Maybe you should have an option to set the cash value of your goods, instead of a % of the current price)
Also: the computer buys goods in increments of less than 100, is this possible for a player to do or is the computer cheating?