In such an environment, a player with sufficient resources can effectively prevent trade among others, through threats, coercion, dumping, buying and squelching potential competitors, etc. Monopolies are especially capable of such tactics. This is why we have anti-trust law. Over the last ten years, Microsoft has effectively prevented trade in operating systems by secret pricing structures and threats of exclusion to manufacturers, in productivity software by closed, rapidly changing file formats and coerced bundling of Office, in browsers by dumping Internet Explorer and corrupting Internet protocols. These actions have manifestly harmed consumers, the software industry, and ultimately the U.S. economy.
Customary law maintained by the self-interest of economic actors is quite sufficient.
Perhaps examples prove that law is not necessary in every case, but they don't prove it's unnecessary in all. You can't claim that Adam Smith's invisible hand will ultimately prevail and right these wrongs retroactively. Clearly what is needed is more effective law and better, more timely enforcement. Lastly, the First Amendment is in fact a regulation on the behavior of Congress. It prevents Congress from making laws that restrict certain freedoms of individuals, the press, etc.