They say wisdom comes with age, by now, I should be wise! Confidence however, comes from within yourself. Days when I am dressed well and my hair is behaving, makes me feel more confident on the outside. Being knowledgeable about my job and optics makes me feel confident on on the inside. Your confidence, or lack of, shows to those around you. When you are feeling sure of yourself, people around you will also feel it.
Selling is as much about being confident in your abilities and knowledge as it is about the product. Sure, there are always some products that "sell themselves," but much of our product is invisible to our clients. They cannot see design technology. It is how you convey it that will sell it.
Always be sure of yourself, it works every time!