Agents and brokers of any kind, in fact. Anyone who thrives when people are in the dark is in ever more danger of working in the bright light of transparent information.
Pretending that you offer the lowest price on a commodity, for example, is a lot more difficult when anyone who cares about the price can easily look it up. Fighting to keep the content of your course a secret, to pick another example, isn't sufficient when a similar course is available online. The minute real estate listings went online was the minute that it was no longer sufficient that a real estate broker merely had information about real estate listings...
Information is in a hurry to flow, and if someone comes up with a better, more direct, faster and cheaper way for information to get from one place to another, they will eliminate your reason for being.
The alternative, while difficult, is obvious: provide enough non-commodity service and customization that it doesn't matter if the ideas spread. In fact, it will help you when they do.