I once had a friend tell me that he could sell me a $3000 vacuum cleaner.

"Really?" I said. "I don't think so. I know vacuum cleaners don't cost that much."

But he was certain of it. He'd sold dozens of these vacuum cleaners. His success rate had been tremendous. He believed they really were worth the money. The evidence really indicated that he could sell anyone a $3000 vacuum cleaner.

At this point... I really don't want him to try to sell me a vacuum cleaner. Or, in fact, to sell me anything. I'm scared he could get me to part with my money way too easily. That could be very bad for me!

Moral of the story: all charisma and salesmanship is, to some degree, a threat. Basically all people will be ok with "How to make a good first impression," but "Subconscious tricks to make everyone want to buy your product" is starting to sound a little sleazy, and "How to tap into neurochemistry to make your product addictive" is probably going to scare people. People get squicked by the thought of how World of Warcraft or McDonald's manipulates their reward circuits.

I think some analogous dynamics hold when the product you're selling is yourself.