Barriers to entry exist with regards to any entrepreneurial endeavor. Are they natural barriers or government created and enforced barriers?
Well there are obviously both. Significant barrier to entry makes it far more likely for cartels to form, whether this is artificial or natural. People here seems to have this idea that cartels are going to jack up prices to monopoly level. That is really fun in theory, but it does not happen in reality. Price fixing is harder to recognize in real life.
My point is pretty simple, I don't see how price fixing can ever be a good thing when significant natural barriers to entry exist: lets ignore government constructed barriers.
Anti-trust laws are some of the most politically motivated and arbitrary laws to date. They establish trusts, even going so far as forcing every single one within this country to accept a piece of paper at current value for debts past or present, then have the nerve to offer a "solution." Even if trusts were an issue, one need not look too far to see that the USG does not have the American public's interests at heart and instead it operates under a system of crony capitalism and theft.
You are quite right that the FED is a government supported monopoly, but I really don't want to get in discussion about currency, there are a million other threads discussing that particular monopoly. But most anti-trust laws are more arbitrary than politically motivated. Laws creating and supporting cartels are generally not considered anti-trust, as bad as those laws might be. Peanuts, Sugar, etc.
No matter how a business operates a case could be made that they are acting in a monopolistic manner. There are natural mechanisms in place that would largely prevent the 'cartelization' of various industries. That is, without government granted privileges and special favors.
The natural mechanism you mean, at least in high barrier entry, is the incentive to cheat. Certainly a grim strategy model is not realistic, but there are certainly also mechanisms for cartels to punish cheaters.
It has been a while since I've read up on the subject but Tom Woods, Jr. has a lot of convincing stuff out there, Tom DiLorenzo has done a lot of work with regards to this issue, and Andrew Napolitano has written about various ridiculous cases of Congressional violation with regards to this in a few of his books.
There have certainly been a lot (most even) very bad anti-trust laws. I just don't see a good libertarian answer to price fixing....and also patents, which I will discuss later.
Slutter McGee