You maybe forgetting something?
The salesman first has to create a need, if there isn't one there already.
This is often done by using fear, nicely nuanced, of course. Fear of something, anything, in the mind of the punter.
Then the salesman puts forward his expert advice on how to overcome this fear, by buying his product.
As said already, he tries damned hard to give you faith in his product, and only his product.
He shows how you will be better than most everyone else by using his product.
He closes the sale; you sign: you have bought the product....
Salesman, Politician, Evangelist; all the same, conmen in different clothes.....:D
Don't forget the 'freebie' there's always a freebie before you get sucked in.