Greece was mainly limited to grain/wheat production, which they decided to regulate. By limiting the import of wheat, they protected their own goods but allowed the import of other goods and food.
By purchasing grain from a special buyer (sitone) they could control the price of grain/wheat. This was especially importing during shortages of food and drought. By buying grain for less money, and trading for export for more expensive amount of money they could protect their production and trade of other goods, as well.