is there anyone?

I'm not a consmetics salesperson. But I think sales skill are also the same in more business.
Summary as follows:
First, you must understand the thing what you sales, if you don't understand about the thing, the customer how can believe you.
Second, you must understand the customers' need. For example, money, hobby, profession and so on.
Third, you must often update your information. The period is often change, so your knowledge must to follow it.