Secondary goods

So I propose a secondary goods market. The secondary goods production is made by a feudal age building. In castle age, you can start trading and using secondary goods via a castle age building.

To use secondary goods, you simply need to have secondary goods stockpiled. The rate at which you use secondary goods is decreased by 10% for every type of secondary good you have, up to a maximum of 80% decrease in the rate you burn through your secondary goods. The castle age building spends secondary goods to generate a constant stream of gold, stone, and cheap units (like archers, horsemen, and spearmen).

To aquire secondary goods, you must trade with multiple civilizations and make your own goods. You can produce 1 type of good as your civ, and every time a trader returns from another civilization, they drop off 3 secondary goods that civilization is capable of making.

So, what secondary goods are there?
Religious figurines
Prayer beads
holy texts

The list is endless. This would allow for more complex economies to emerge, especially if traders only generated secondary goods, and not straight gold.