playing as Venice I used the textiles in N Italy (two provinces, four resources) to train merchants. I had forts controlling all the passes from europe into N Italy which created an "agent net" to protect them as they levelled up.

then when I had a few with decent skill i left one hanging around to protect the tadpoles against any decent merchants to come wandering by.

If you are aggressive enough with merchants you can send a wave of high-level traders out across europe and eat up all the good AI merchants within a couple of turns

then all the new ones the AI pumps out will be less skilled than your guys and can become training aids themselves