I do realise that lead information is sold, and I've had enough offers to sell my own users (which I've declined) to realise just how prevalent the practise is.
LinkedIn sell a fairly complete business dataset. My point is that a lot of people might imagine they could do this, but probably don't really believe that they are doing this.
Then when you add in Intro's almost constant tracking (vs occasionally accessing one of the sites that sells your data - or LinkedIn on the web) it is easy to see just how complete one would be making that dataset.
I'd say that most people don't really understand believe that this happens and how good (if that's the word) that dataset already is.