Selling relies on the principle of service because you are serving someone when you are trying to help give them something that will fix a problem or just something that they need or want. When you are actively trying to sell something to someone you want to sell something that will benefit the other person because that will help them want to buy what you are trying to give them, you don’t want to try and sell something to someone they don’t need and they’re not looking for because then they won’t want to buy something from you and they won’t want to buy something from you in future which will make it so that you don’t earn a profit.
When we are serving somebody else we are helping them with something they can’t do or we are giving them something that they wanted but didn’t have the resources to get, which is also the principle of selling something to a customer. The reason a customer goes to someone that is selling something is because they need something or they want and are willing to buy or trade something they have for the object they want.
When selling something is done right is almost the same thing as doing service to somebody else, when we are selling something to somebody we are relying on the principle of service because what we are trying to do is help and serve them so that they will give us money because you need that to go and help or serve yourself, or if you are trading an object for another object it is the same case.