Yes. I agree with you. Dependence purely on body language will prove counter productive. But one thing must be remembered some people are capable of speaking with a straight face. No feelings, no emotions. It's from such customers the Salesman should be cautious.

Why should a salesman be concerned about the body language of the customer? He should concentrate on his own verbal language to lure the customer.  A customer has a purpose and he has some budget. His body cannot talk more language beyond his needs and budget and it is often limited to a sign of surprise, hearing the price; or shine in eyes seeing the quality of the product. Ultimately he walks out, buying what he prefers; but not what the salesman prefers.  

Body language reveals what verbal communication fails to deliver.  A customer always looks for best returns for his investment. When a customer knows that the price of a product is beyond his budget his inability to get his product is very conspicuously expressed only through physical gestures.

Yes our body expression tell almost every thing. We does not need every time some word to explain our emotion or what we want.  

Words may lie but not Body.

Body language used properly could be of great help in trying to communicate with other persons, but you should be aware of the culture under which the other person or persons have grown up. In some countries the touch of a person, particularly if it is a woman, is not welcome, in other some gesture could be very offensive, in others to stay close to a person during a talk is not welcome, and so on and so forth.
Taking these limitations into account, it is of great help the use of body language to transmit an idea to others, to show that you are interested in the conversation, that you understand the subject that you are discussing, that you are ready to cooperate, etc.