Theoretically, nothing. The market will work itself out.
Practically, ensure the market is operating as best as it can - provision of information, reduction in assymetric information, ensure property rights are fully defined.
A market can be free with regulation. If there wasnt any regulation, all markets would end up in anarchy.
The right amount of regulation creates the perfect equilibrium of a perfectly competitive market and one that maintains a level of acceptable integrity, without suffocating it with over regulation.