The successful ones are the ones that truly believe in what they are selling. They say what they think and they can rebuttal any obstacle because they believe in the product inside and out. They are confident self-assured and their is an air about them that we all get intoxicated if we stay in their orbit for too long. Same goes true in the opposite scenario. Somebody who is thinking this car is a dud I just got to unload it will come across as nervous or that over confidence that makes us run for the hills and never return.