The statement is an analogy, a negative one, because the salesman man is not a snake (Analogy) and the statement is not really a happy thing (Negative)
In this case, the comparison between the salesman and the snake is an example of an analogy. An analogy is a comparison between two things in order to examplify or clarify. Moreover, the tone of this analogy is negative, as the qualities that are being compared are undesirable traits. The salesman is "smooth" and "deadly," which are characteristics that are likely to make the customer uncomfortable.
