How does trade profit work on Age of Empires 2?

Answer 1

When you send a trade cart to a market not belonging to you, or a trade cog to a foreign dock, it repeatedly goes back and forwards between its originating market or dock to the one you told it to go to.

On the outward trip, you don't earn any money, but when your trade unit reaches a dock or market, it picks up a certain amount of gold based on distance, and returns to your dock. You only get this trade profit when it returns to you though, same as with gold miners. Just think of it as a gold miner with 0 mining time and the mining camp is you dock or market and the other market or dock is the gold mine.

The further away a dock or market is, the more gold it will give you (you can check by selecting it), but be careful, as this is determined by map distance, not the distance you have to travel to reach the dock or market.

It's worthwhile going to docks or markets further away, as you get proportionally more gold than a shorter trip. At the market you can also research caravan (doubles trade speed) and cartography (so you can see your allies line of sight, and so where their market is, as well as any of you allies' units or other buildings).

Enemies will still attack trade carts, so make sure they're not going through a dangerous zone, or you can stick a few monks next to the market to heal incoming units.

The more trade you have, the higher your profits, but be careful, as too many will mean the place overcrowds and they all get stuck in. It should be noted that you can trade with enemies, but in later ages they will wall up, and destroy your trade units, so it's not advised.