Confidence is key to selling. If you lack confidence and are meek, it doesn’t matter if you have the best product and it’s at a great price. If you leave any doubt or confusion in the buyer’s mind, you’ll lose them to more confident competitors.
When you lack confidence in your own product, people will find reasons (real or made up) as to why it’s not a good purchase.
Know your product inside and out. Know the competition’s product. Understand the market in general. Be positive, talk clearly and concisely, and be energetic (but not annoyingly so). Dress according to what is expected of the product you’re selling and the people you’re selling to. When in doubt, slightly overdress as opposed to being underdressed for the occasion.