Buying and Selling is:
O Economy
O Socialism
O Trade
O Capitalism

I'd say Capitalism

Explanation:

It is part of economy as a whole, but it is also capitalism.

Definition of economy: the wealth and resources of a country or region, especially in terms of the production and consumption of goods and services.

Definition of capitalism: an economic and political system in which a country's trade and industry are controlled by private owners for profit, rather than by the state.