I'm not familiar with any charlatans or scammers being successful by pretending to be smarter than they were. People pretending to be smarter than they are, are usually pretty transparent.
They are if you're smarter then they really are.
I suspect this is just availability bias, though, do you have any examples in mind?
Well, there's Yvain's tale of how he was almost convinced by Velikovsky's pseudohistory.
It seems you are using 'seeming smart' as interchangeable with 'convincing' or 'persuasive'?
However, these are quite independent. Someone can easily convince me of something, without my thinking they are more intelligent than I am, and without convincing me that they are more intelligent than they are.
Consider a 'smooth talker'. I think people generally recognize that these smooth-talkers are more likable and persuasive on any topic, but there is no necessary correlation with having a higher IQ. In fiction, there are extreme examples like Forest Gump (low...
I've never heard of anyone saying "I thought that person was really intelligent, but they turned out not to be", and when there are scandals about people with fake credentials, they don't seem to come from people with fake credentials making mistakes-- instead, someone checks the history.
It seems to me that you can find out a lot about people's intelligence by talking with them a little, though I've underestimated people who were bright enough but didn't present as intellectual.
The real problems are with identifying conscientiousness, benevolence, and loyalty-- that's where the unpleasant surprises show up.