I think this is a try and error thing. There are very few merchents who knows what good, if sold to a particular port, will fetch the highest price.
I would suggest these guide lines when being a merchent:
1 > Be aware of the goods in each of the ports so as not to bring a good that's being sold in a port as that's not profitable
2 > Be aware of the neighbouring ports as well. They affect the demand for some goods.
Some ports do not give profits even though the good that you're selling is not found in that port.
3 > Being aware of what good is in demand for a particular port(i.e. via public chat where you see msg about demands in all ports).
It changes every 2 hours (or less)
4 > Distance and Time matters! The further you go, the higher the price you can fetch for your goods.
But keep in mind of the time you spend on each trip + purchase of provisions + pirate occurance
There could be more guides lines that others will provide, but I think these should do.
If you want fast cash, I still prefer fishing in Alexander and sell to Athens; 1 Anchovy = 98, 1 Swordfish = 128