Buy & Sell, A phenomenon on which the world works. If you think about having any commodity or item , the first thing which comes to your mind is where and how am I going to buy it. It can be from a store or online free classifieds. Selling works on the same principle with added questions to whom a commodity or item should be sold keeping in mind other factors such as profit expectation and trust.
It has a common factor which drives them which is money involved. Years ago when transactions happened to be purely an exchange of items called barter system, the terms buying and selling were not used.
It was fairly on the system that you give me something and I give you something in return which used to result in a formal transaction. The system lacked freedom as a buyer and seller which is why money changed the game as it was introduced.