Snake oil salesman refers to someone trying to get you to buy into something that is supposed to help or do something good but really is nothing at all like that. In fact, it may even be harmful.
Where did the term came from ?
During ancient days, medicine salesmen used to sell snake oil as a curative, as a health benefit, a cure for all; when it of course did nothing of the sort.
Snake oil salesman : Are they genuine ?
The salesman usually knows very well that what he is selling is completely useless, and even harmful. It isn't like he is naive or misguided, he knows the weak points of mass and hits it hard there.
Politicians as snake oil salesman could be the most common example.