In my current Portuguese game I control Venice, Milan and Genoa while the Pope controls the rest of Italy. I decided I would use merchants heavily in this game since Timbuktu was relatively nearby, so I now have three very experienced merchants sitting on the textiles resources in Northern Italy, hoovering up any merchants which come through.

The Pope seems to be determined to get his merchants out towards the silver in Vienna, and as such every turn he has been training two merchants and sending them North through my lands, where my veteran merchants promptly snap them up. Thus I'm making hundreds of florins every turn from them, plus the money from the actual trade resources.

I've never really bothered with merchants before, but they seem to be very worthwhile for Portugal right from the start since Timbuktu is so close and the AI doesn't ever seem to bother with it. In a previous campaign I once had a single Turkish merchant wipe out my entire trade operation in Dongola, but I've never seen the AI try to get merchants to Timbuktu.