I have read here that trade revenue is crucial. But I find trade revenue pales in comparison to the kind of "tributes" I can get from selling techs and trade goods.
I get like 1600 bc in "tribute" while only about 150 in "trade" on my economic control screen.
Trade revenue often isn't as crucial as the good will you get trading with an AI. They like you MUCH more if you are trading with them, and especially if they are stronger than you, you want them to like you. Selling tech for cash is one of the easiest ways to keep your economy up and win, at least up until "masochistic" difficulty level, when nearly all the AIs just won't give you money any more. Until then, I mostly sell tech for cash and use trade for secondary cash and to make the AI not want to attack me while I'm still building my social and resouce foundation.