What's one quality that sets apart a good salesman from a great salesman? Confidence. Confidence in yourself and confidence in the product you're trying to convince people to buy. With confidence, a great salesman can sale anything to anybody.
But where does that confidence come from? It starts with knowing your product inside and out. You have to be so well versed in that product that you can make outrageous claims about it and then back up those claims with real demonstrations. It doesn't matter if you're selling car wax, solar panels or even acrylic bongs. I great salesman can look you in the eye, tell you what they're selling and then give you a million reasons why you need that thing.
Another thing that sets apart a good salesman from a great salesmen is great salesmen never intentionally set their own product on fire. That is what a bad or "not good" salesman would do.