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.
I'm pretty curious how they advertise the fact that they do something like this as well.
Some time ago on HN I remember reading about a company that embeds forms on websites. So if I filled out a contact form on Site A, the third party collects the information, stores a cookie on my computer. Then when I visit Site B, the cookie uniquely identifies me, and the third party company gives my email address to Site B, even though I didn't fill out a form.
Is this what you're suggesting marketo does?