is that when you are crossing the map keep an eye out for icons .
Gaining exp points through the sword icons allow you to level up your merchant and other party members as well as the meeting new people icons allows you to pick up free auxiliaries or trade 5 water for an auxiliary.
When planning my route I check for cheap goods on my route and expensive goods. Pick up the cheap goods and sell at towns where the goods are expensive. The world refreshes every couple of game days and prices changes. It's not frequent enough to throw off my trade route plans.