You don't have some unique immune ability. The OP said he approached the vendor first for some sales advice, and in the end the guy got him to buy his wares.
But the OP also admitted he had no particular interest in sales - a cue to the huckster that he was dealing with a clueless innocent who could easily be manipulated by the tricks of the trade.
This leaves open the question of why the OP approaced the salesman in the first place.
