Two equally presentable advisers can proffer the same advice. One is perceived as a sage, exuding knowledge and wisdom from every pore. The other comes off as a novice or a buffoon. Same setting, same advice, different results. I've seen it a million times.
Confidence is a tonality thing.
Part demeanor, part attitude, part style. It's hard to define explicitly. Like good modern art, you know it when you see it. It involves having experience, being a good listener, thinking about the response before the response is made, acknowledging strengths and weaknesses and, most of all, being honest with the buyer. For example, there are times when it doesn't make sense for the prospect or the client to buy the product.