On the subject of Timbuktu:

When negotiating trade rights, map info and/or alliance with the Moors, it's always worth seeing if they've occupied Timbuktu and are prepared to sell it to you.

In the earlier part of the game (pre-turn 40 on 2 years per turn), there's a good chance that, if they have bothered to occupy it, they have only just done so and won't have had the chance to build it up. That being the case, they'll be more prepared to sell it to you for next to nothing - often alliance, trade rights and map info will do.

Once you've got your hands on it, build a town hall and a grain exchange, then pump out merchants. Total investment will be 3,400 for those buildings and four merchants but you can realistically expect to be earning half that back each turn in no time at all. No need to send an army or have a governor wasting away in the wilderness. Being so far from anywhere else, you have less to worry about from predatory merchants, too.

And, if you're a Catholic, you'll need to build priests but they'll be surefire Cardinalate candidates, so you win on that score, too.

I've done this as most of the major Catholic factions and it works a treat. I don't tend to bother with merchants otherwise, as I simply can make enough money without the bother.