Just as a heads up:
There appears to be a variety of different "Key resources" by faction that will generate different levels of income from the same resource.
For instance:
As England the Timbuktu mines are the ultimate resource for income from merchants - I cant find anything else that comes close.
However when playing as Venice this was not the case - the ultimate income earners were ivory and slaves. I originally thought that this was simply down to the distance-from-capital modifier being different, but having conquered England as Venice and moving my capital there it transpired that ivory and slaves still generated more income than the Timbuktu gold mines.... and that the venician income was lower than English income despite having merchants that (visible) were of the same level.
Unknown elements in the equation though remain the fact that merchants can reach levels higher than that shown on their cards - just as with assassins and similar level 01 is not the maximum, so it may have been that I had a level 15 or 17 on the Timbuktu gold mine when playing as the English and only a measly level 10 as Venice.
Its worth keeping an open mind though and checking out various resources when playing as other nations though.
The top resources "in general" do appear to be gold, ivory, amber, silks, slaves but I'd experiment a bit with various factions before just aiming direct for Timbuktu and assuming you will get "the ultimate income".
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