I tend to just pick a route to hit a bunch of different towns. I buy whatever seems cheap. If it seems on the cheap side, but by a small margin, I just buy a few of that item. Maybe even just one to either use as a reference for price point or bump the morale from food variety if the trade good is food. Then I sell at the next town or the one after that if the price margin is good.
I prefer playing as a Battanian, so I usually spend at least some time in Battania. In that region (including the neighboring Vlandian, Sturgian, and Imperial towns) I've found that I can usually find neighboring towns that allow me to make a 300% gain on iron ore, hardwood, and meat. Notably, butter and cheese bought at Poros sells near triple or more at nearby town, name starts with a Z, to the west. Good starter coin.
I've seen much better gains travelling farther distances for butter, salt, oil, wine, and horses, but almost every time I plan big trips for a single item, the price drops by the time I get there because a caravan unloaded before me.
I get the most money trading by hitting town after town buying anything cheap, only selling later for at least double, and then betting my gains on any tournaments I find.