As said P2P will also buy CE themselves. No everyone spend cash every time when they want CE.
Even if OP modify the suggestion to make it so you can only buy or sell at a time (only buy or only sell), it can be easily bypassed with multiple accounts / multiple computers / multiple people.
The key to shill bidding is to cancel their buy orders before they get processed. Focus on punishing that behavior.
This game is F2P in a sense that you don't have to pay to create an account and start playing.
Stop misusing terms. And the occupy one too.