Though some of these might seem like the inclusion-criteria is vague, it's really not. If the practioner requires a deep understanding of epistemology to be successful (or, if an understanding of epistemology makes one more successful) in the field, then it's definitely included on this list.

Most of these seem to boil down to "people who lie", but it's more than that. A mentally-ill person in jail will lie, but that's an uninteresting case. The cases below are people who can successfully lie to a decent percentage of the population. The ability to do this requires knowledge (even if implicit knowledge) about how people's "sense of what's true and what's false" breaks down.

In black box testing, you learn more about how the innards of a system work by studying the failures than you do the successes. Human failures such as logical fallacies and sensory illusions give us a great amount of information about how the human mind works, and that applies equally to studying epistemology failures.

Benevolent / non-malicious

• Illusionist (magician)
  Modern magic is very interesting. It's one of the few places where the audience knows they're being lied to (tricked), but they willingly choose to stick around and be tricked. Especially because of James Randi's work, I really believe that magic and skepticism have a lot in common — by performing magic in front of people, it encourages them to 1) realize that there are less benevolent tricksters out there, and 2) actively helps the audience practice the "mechanics of skepticism", i.e. helps them run through the process of trying to figure out how someone might trick them.
• Skeptic organizations
  Skeptics fill the void between scientists and laymen. Real Scientists^TM almost never bother to try to explain science to laymen — science is almost guaranteed to win over anti-science people long-term, because it formally includes rigorous self-scrutiny, and competing systems don't. Also, scientists have much more important things to spend their time on.

  Skeptics are people who (for whatever reason) are unable to carry out real science, so they do have time to talk to plebians. And as popular anti-science sentiment grows, skeptics see a growing importance for their role in educating and communicating to the populace.

  Skepticism (and its evil twin that it's often confused with, pseudoskepticism) are "applied epistemology". The practice of submitting fringe theories to real scientific methods, as well as responding to fringe theorists' responses in a respectful and articulate manner, requires a firm grounding in epistemology, lest practioners feel the urge to start swaying with the breeze.

Unintentionally malicious

• Religious leaders
  I think the people who create a new religion are outright malicious. On the other hand, people who, much later down the line, continue to propogate it are instead just unwary zealots. Nontheless, by propogating the originator's ideas, they're unintentionally causing damage.

Ever so slightly malicious

• Horoscopes
  Intentionally exploit the Forer effect.
• Individuals who are skilled at lying on a day-to-day basis
  See the section of Richard Wiseman's "Quirkology" about lying and lie-detection. It's fascinating, and it suggests that certain types of people are much more practiced liars than others.

Somewhat malicious

• Cold readers (e.g. fortune tellers, psychics, mediums, mentalists)
  In modern times, there's a lot of educational material on cold-reading. Anyone who studies it with the intent to personally profit from it is clearly malicious.

  On the other hand, cold readers usually don't make the mark part with very much money. (as compared to hot readers or con-men)

• Historic creators-of-religions
  On one hand, you can say they did a lot of harm, because they have affected so many people.

  On the other hand, I think that people from earlier times wouldn't necessarily have been aware of the full negative impacts of religion. Also, religion is marginally justifiable (back then) by instilling good practices in the populace. For example, back then, non-kosher foods really did carry an unnecessary risk, and since there wasn't a formal system of education, it was hard to get that message out without self-replicating indoctrination.

  In modern times, however, creators-of-religions are much more malicious. They operate in an environment where the negative impact of religion is clear, where modern psychology gives them a lot of help in exploiting people's weaknesses, and where modern education systems and modern media eliminate the need for indoctrination as a means to keep the population safe.

REALLY malicious

• Confidence man
  Confidence men have to know what people's vulnerabilities are, and they have to really study hard how to properly lie to someone. (this is particularly true for long cons, where a great deal of planning and theatrical stage-work is done to convince the mark to continue giving more and more money)

  Learning how people make mistakes in their thinking, and studying it so thoroughly that you're able to exploit it when the opportunity¹ arises — in my mind, that's the very definition of "malicious skepticism", and so is involved in epistemology.

  1     IMHO, the majority of crimes are opportunistic; they're done whenever the opportunity presents itself. The whole thing about "security is only as strong as the weakest link" means that security is almost always attacked through the weakest link. However, most weak links are temporary, caused by personel slip-ups or personal laziness.