The victim of a scam is often known as a mark[1]. It seems to me that jonhendry was implying that Ferriss is a con artist, using his book to defraud people of their money, by making them pay for something which is worth far less than they think. I am, however, not familiar enough with Ferriss or his books to pass such judgement myself; I'm just interpreting.
Every author has a target audience. Although I find Ferriss' marketing strategies unethical, I think his judgement on the book itself to be a bit unfair.
