I try not to put an inexperience merchant in the hot spot (Europe and around Constantinople, Antioch and Dongola). Trade goods are great, but it is not a good place for merchant with 3 finance or less.

In my Spanish campaign, I found that ivory (4) and slave (1) trade in Arguin and Timbuktu are great. Even better if I own those 2 towns. My merchants make between 300 to 600+ for ivory and about 200-300 for slave and there is no challenge yet. Also, the spice spot in Jedda (spelling?) is great. Much safer than around Antioch. The Egyptian Merchants have been very aggressive around Dongola and Antioch.