Market competition between sellers of products and services for buyer’s money, and between buyers for seller’s goods and services results in cooperation between buyers and sellers when a trade is made.
It is important to note that the distinction between seller and buyer is an issue of focus, as individuals are buyers are sellers, and all sellers are buyers.
They are two sides, so to speak, of the same coin. Sellers trade (sell) products (supply) for money. That same seller will later use the money acquired to buy products as a buyer. Buyers trade (buy) products (demand) with money. That same buyer previously acquired money by selling products and services as a seller.*
The idea that one’s supply constitutes one’s demand is known as Say’s Law of Markets, i.e., that one pays for one’s products with the products one has produced.
 One can also obtain the money from someone else (who produced the products and services) either by voluntary charity or by coerced, involuntary transfer payments (taxation and theft).