If you understand how people tick – how they respond to certain ideas, how they act in certain environments – if you understand the basics, you can, theoretically speaking, affect the way people think around you in a deliberate, pre-determined fashion.
And if you do this in a specific way, you can get people to buy from you.
The system for influencing people is a tool, just like you’d call a knife a tool or a screwdriver a tool. Just like you can use a screwdriver to help you build something or hit someone on the head (might need a big screwdriver if you want that one to be effective), you can influence people to do a variety of things, some that your internal moral compass will consider good and others that it will consider bad.
Usually our expectations are based on what we want to happen. On the other hand, what actually happens depends on what we do, or if your expectations are from other people, what they do.