First I thought, trade is exploitation !! But wait, framer alone makes 10 kg/day veg, 2 liter/day milk, milkman alone makes 2 kg/day veg, 5 liter/day milk. If farmer makes all then total in 2 days 10 kg veg, 2 liter milk. But if farmer focus on farming then 20 kg veg, and milkman 10 liter milk. If they exchange then both have more !!
Only limited trade as making all increases capacity, no dependence !! Learning !!