Sales are hard. You have to know your market, and to know how to work the market. You have to know your customers, existing and potential. You have to know your leads, and how to follow up on them.
And, (this should go without saying, but you’d be surprised how often it doesn’t), you also have to know what you’re selling.
Factor in the constant changes to the market (new models, new trends, new fads, new technologies, new competitors) and the human element (talking to people and closing deals), and you’ll come to the same conclusion we began with: sales are hard.