Just send an army down to timbuktu and take it and train your merchants there and trade all the gold/ivory/slaves down there, the AI never goes there(with the exception of maybe the moors) but if they do having a watch tower north of timbuktu will give you like a 7 turns heads up.

I was making 4k florins per turn just camping all the spots at timbuktu and i didnt even skill up them anywhere or anything, just trained the merchants in timbuktu as they died off naturally.