Dunning–Kruger effect

An effect in which incompetent people fail to realize they are incompetent because they lack the skill to distinguish between competence and incompetence. Actual competence may weaken self-confidence, as competent individuals may falsely assume that others have an equivalent understanding.

Your customers aren’t going to know some things, and they aren’t going to know that they know it. You, as the seller, need to be aware of your customers’ blind spots. Why? Because they themselves aren’t aware of their blind spots, and therefore don’t know how to overcome them. The greater awareness you have of your customers, the better position you’re in to sell effectively.