Rude buyer towards sales person - seeing the other side

I have had so called dealings with many sales people and also the managers. It really gets funny when you strike a nerve especially when you know the figures and know as much as they do.

They seem to get attitudes quickly when you know your stuff. I have walked out many times when they don't follow through with what they say once they put it on paper. I don't see where the buyer did anything wrong, he was negotiating. They shouldn't be doing it where others can hear them anyhow!