Many of the best-known dealers probably have some people skills and maybe even some integrity (or at least the ability to feign integrity), otherwise they wouldn't be very successful. However, when it comes to small-time dealers, I've noticed that many of them have personalities that would prevent them from finding any kind of success in the corporate world—which is probably why they're self-employed.
It isn't as easy to get away with things like amateur resto and trimming these days, but the sleazebags have found other tactics; like getting fake sales into GPA to pump up prices, shill bidding on auction items; buying the top-five graded copies of a scarce book and cracking out the top four and sending in the labels so they can sell the fifth-highest-graded copy as the highest-graded copy, only to turn around and slab the erstwhile fourth-highest-graded copy in order to sell THAT one as the highest graded copy, etc.
There are con artists in every hobby and every business, though, so why should comics be an exception? Most people are restrained somewhat by a conscience, but these guys are conscienceless; they're like flesh-eating bacteria that are resistant to antibiotics. You can try to mitigate the damage that they do, but you can never really get rid of them.