What are the efficient ways to spend gold coins in Age of Empires Online?

I'm new to the Age of Empires Online and I couldn't find a good guide about its economy.

What are the efficient ways to spend gold coins? I'm not talking about quests; I'm talking about the capital city.

Ultimately when you reach level 40. You will be looking for mainly three things to buy with gold coins:

1) Buying legendary gear (gear with orange text) from the Legendary Armament shop in Cyprus (the city will unlock for you as you progress through the game)

2) Buying random chests from Moe's Mystorium in another player's city (provided they have the shop), hoping that you get good stuff from it.

3) Buying from the trade channel from other players. You often have players selling epic/legendary gear/recipes/materials in the trade channel. Ask them for a trade.

This is all I am aware of. Some stuff cost more than others.