In our interactions with people only 7% of the communication actually takes place with our words. Let me state that again. Of what people interpret us communicating to them only 7% comes from our words. The rest comes from our appearance, actions, tone, volume, speed at which we talk, smell (yes, what smells are coming off of us sends a message) etc.
If you want to be a master negotiator/person of influence and have a sense of many of the ways that we are influence in our daily lives and aren't even aware of it check out this book by Robert Cialdini called Influence: The Psychology of Persuasion. He has another book out that has been recommended to me called Influence: Science and Practice. I have not read Science and Practice yet but it is on my list.
When you are negotiating with people it is not the words that you are saying. Logically it may look like a no brainer to sell their house to you or to move into this property that you own. But if there is no emotional feeling tied to that decision they aren't going to make it. It is the emotions that you are stirring up inside of them and the feelings that they are feeling that makes people jump to a buy or sell decision.