I agree that libel and slander shouldn't be illegal. You may say, "but it is giving false information!" However, a true libertarian accepts that there are lies and false truths out there, and that it is the individual's responsibility to unravel these lies if necessary. People should not depend on the government to take care of these matters, because the government simply can't, and ultimately, the people who falsely believe they are protected will only become exploited by those who slip through and lie. People ought to be cautious about what they read.
It is hypocritical that some libertarians who oppose libel and slander would prefer financial markets to be freer, when doing so would install a similar type of risk. Freeing the financial markets will cause people to be more cautious about investments. Instead of relying on government agencies, watchdogs and bureaucracies to make sure the CEO isn't doing some dodgy things with your money, the investors will take it into their own hands. They will invest less, unless the CEO uses company transparency (security for the investor) as a bargaining chip. This is the way things should be.
N.B.: In this light, government regulation is often corporate welfare. It serves to promote investments by putting some degree of government-backed guarantee on them. But what is the cost? The cost is a hidden inflationary tax created by a government that authorises the central bank to print more money to fund such regulation. In this way, such government regulation has served the rich and punished the middle class.