The relationship between the customer and the sales clerk is adversarial to the extent that the sales clerk is trying to find out what the customer needs, the depth of her committment to spend money satisfying that need and what she will accept to fulfill that need.
That's a lot of information the clerk must gather before he can close a sale. The easiest way to gather that information is to engage the customer in conversation, let her talk, and remain attentive.
If the clerk is smooth at engaing the customer in conversation, the relationship becomes less adversarial. In some cases, it can resemble the relationship between a therapist and his patient.
Your idea of the clerk's job description assumes that he works for you. He works for the store, and his job is to move the merchandise at the highest price and lowest cost.