Interesting question.

A skeptic, critical thinker might be tempted to believe that since the methods by mediums and the like seem so inherently dishonest that nobody could possibly honestly believe in their own abilities. However, it's easy to forget that these people are not holding their methods to the same level of scrutiny that we might take for granted.

If someone can be fooled by a skeptic to forget/forgive all the 'misses' ('I'm getting a name like Jim or James, does that mean anything to you?' 'No...' 'Interesting, well, look out for that.') but remember the 'hits' ('This person... I'm seeing the colour blue associated with them...' 'That was my husband's favourite colour!') then it's not too much of a stretch to imagine that the medium could fool themselves in the same way.

Try it now... let your mind go blank and then allow some thoughts and images to just come to you. It's easy to see how someone who is already predisposed to believe in the paranormal might believe these images are transmitted from elsewhere and have some mystical significance. Perhaps they start to believe they are psychic in some way, or in contact with the dead. They test this with some friends and they probably experience quite a few 'hits' because they know the friends quite well. This reinforces the delusion, and when they start to speak to people they don't know, they are still not critical enough of the process to pay much attention to the increase in 'misses'. Maybe they just choose to believe they were more naturally 'in tune' with their friends and family.

They probably get better over time as they learn the right questions to ask - and they might get help on 'cold reading' (the art of making vague questions sound specific) from others, without realising what they are doing. Any explanation that reinforces what they want to believe can be used, anything that counters it can be ignored or explained away - effectively they could be victims of exactly the same fallacious thought processes that their clientele inevitably go through.

In short - no, it's not obvious that all claimed practitioners of these 'skills' are dishonest people. Although there is a huge amount of money made by dishonest practitioners, it is easy to imagine scenarios in which many so called mediums, clairvoyants and fortune tellers are simply mistaken because they refuse to employ doubt, just like their customers. Whether this is that much better than plain dishonesty is another question...

The fact that the roots of anything can be traced to some past dishonest occurrence doesn't mean that everyone who is involved with the subject since is therefore dishonest, so at face value the answer to your question has to be no, unless there is overwhelming evidence to the contrary.

Then consider the human capacity for self-delusion, insanity, rationalisation of phenomena, and you have many scenarios in which it's conceivable that someone could be mistaken in the belief that they have "mystic powers."

Finally, consider that James Randi's Million Dollar Prize has had applicants that fit your description, none of which have succeeded. Is it possible that they are submitting to the test dishonestly and hoping to win? It is, but they know the test will be conducted under rigorous scientific conditions, and so a fraud is not just unlikely, but EXTREMELY unlikely to win - it would have to be masterful deception or an amazing stroke of luck, while a negative result makes them look incompetent or fraudulent (although of course the rationalisations immediately ensue). It seems reasonable again to conclude that these subjects truly believe in what they do, despite the implausability of the claims and even the trickery they may resort to to bolster their results.

Similarly, ghost hoaxers are sometimes true believers who feel that "planting evidence" is justified since it brings attention and support to the phenomena they "know are real."

Of course not. It is implausible, if not impossible, to demonstrate that there is intention to deceive for “ALL” these practitioners.