In my opinion that refers to everyone lives by exchange something all the time such as an idea or skill. Although we are not a seller, we can offer our idea or skills to others and they receive them for making their profits. This quote tell me that if we are not offer something to another, we are not gain anything back. When everyone offer something to another, they often want something back for example, money, thank, opinion and agreement. A job where I would not expect someone to be selling anything is a doctor. A doctor sells diagnosis that helps a patient to get a better health.