First, they pillage (rob) the capitalists, larger farmers, nobles, real estate owners, all others having money or wealth. Later, they are selling natural resources. The Soviet Union was selling crude oil, natural gas, metals etc., for food and technology.
They sell all they can. Romania was even selling people for money: citizens of German origin to Germany. Many dictatorships do that to survive, not only communists: German Nazis in WW2 robbed Israelites but also pillaged all conquered nations and territories.