There is a lot that people are unware of, the most important would be things about themselves and other fairly obvious things we just fail to consider. There are two things I would point out here: Your vulnerability and the fact that most people who do this do it for a living/profit.
This reminds me of an experience I had years ago at one of these "wholistic" fairs that had a psychic section along with everything and the kitchen sink. The times I've gone I typically just kind of walk through and don't particularly have much interest except to see what's out there, but this time this woman in the psychic section called me over. Actually, at first she asked a young girl, who must have been her apprentice or something to talk to me but she refused, which is kind of telling. Anyway, the gist of what she said was that I had a deep pain, and I can't remember whether she explicitly stated it has to do with romantic stuff but basically that's what ended up happening when I got home, and it felt very deep and seemed like it came out of nowhere. It's a vulnerability. Even though I'd been around the block and didn't really take her very seriously during this little free reading, it's clear something happened. I might add too that I was in a good mood and am aware enough of the way I carry myself that there wouldn't be any obvious cues that most people can read. I still didn't believe she's in a position to really help me (or even really have my interest at heart) as I tend to find these people don't address the deepest level of self, and since I'd already gotten to the point that I can ride out very strong bad feelings that's what I ended up doing. But like I said it was a lot more painful than there's any good reason for even if there's something deeper she didn't touch, and not everyone would have this perspective.
So basically I wouldn't just be concerned about wasting money on a fraud who doesn't do what they say they can do, but if they're able to make a living at it they must know how to do SOMETHING. I'm reminded of the idea of inception from the movie. And consider the basic relationship: You are seeking someone out who is supposed to be aware of something you are not aware of, this in itself puts you at a disadvantage and you have to trust or believe them. And again, they may be able to do something you're not aware of, just not necessarily what they say they are doing. And it may not be a matter of just "reading" anything, something may be done to you in some way.
My advice in a nutshell is that if you have any doubts at all anywhere about the integrity of someone who does anything like this, including teachers, ministers and so forth then you're better off dealing with things yourself than put yourself in the hands of someone questionable in anyway. And I don't mean to frame it in some fearful paranoid way, it is quite possible that the person doesn't really know what they're doing but do stumble on something real in the process. But in that case, it's basically just the blind leading the blind anyway.
Excellent post. I never really thought about it quite like that before.