Once you have the language of the body mastered you can use these verbal skills to further improve your chances of a successful outcome.
Ask the person what they are selling and what they want for it or what they are offering for whatever it is you are selling. This puts them in the position of having to show their cards to you while you take the time to observe their body language and what they have to trade.
If they get uncomfortable or irritated by your questions, this lets you know that there may well be something that they are not telling you about the item.
The more questions you ask the more chance you have to learn something that could lower the price. Keep in mind that this works both ways. If the seller is a skilled merchant, they can pull just about as much information from the questions you ask and the response you make to their answers as you will get from their answers.