User blog comment:JAlbor/Assassin's Creed: Black Flag Q&A Opportunity/@comment-4055238-20130812051039/@comment-6517-20130812094631

There's the trade routes, which go over land and sea. You can also sell on anything you get from hunting animals. I actually found that, as is usual with any game with a built in economic system, by the end, I had a significant surplus of money.

(At least in AC3 I found myself always wanting more, since the pound sterling is my native currency, and I like the idea of being rich.)