Abide by the Golden Rule
Practice the Golden Rule in all your interactions with others. Do unto others as you would have them do unto you.
Source: Sales trainer Brian Tracy
Think about yourself as a customer. How would you like to be treated?
Obviously, you would want salespeople to be straightforward with you. You would want them to take the time to thoroughly understand your problem or need and then show you, step by step, how their solution could help you improve your life or work in a cost-effective way.
If this is what you would want from a salesperson selling to you, then be sure to give this to every customer you talk to.