He knows what forces influence the prices, what events affect them.
These are the types of questions you will have to ask yourself constantly. Or is it worth to buy illegal herb in the out of reach swamps where it cost's next to nothing, and risk smuggling it into the city where you can sell it for an amazing profit? A good merchant needs to know everything about the market, he is always on the lookout for a deal, always listening, always engaging and deepening his knowledge. But is it worth carrying them for so many days, risking robbery, moths and taking precious space that you could use to transport something now and make a profit immediately? Regain your riches, restore your honor and punish those who have wronged you.