The opportunity cost of a choice is exactly the next-choice, the choice that is foregone. For instance, if a choice is between a trip to Caracas and a season's subscription to the Lyric Opera, and the trip is chosen, its opportunity cost is the season's subscription. For any commodity, everything except the price and quantity held constant, as the price rises less of the good will be purchased, Notice that this goes counter to the often expressed idea that sellers can raise their prices without limit. For any commodity, everything else constant, if the price rises suppliers will try to bring more to the market, and vice versa. Market - Any situation in which buyers and sellers can communicate.
Choices take place here.
Almost all societies do it. Required it there is specialization. A most important point is that an exchange only takes place if both parties gain from it. The classic basis for trade. It says that one party need only be relatively better that the other at producing some good for mutually beneficial trade to take place.