The conditions are constantly changing, and supply and demand for different goods with them.
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? And yet the job is harder than it seems. A good merchant surrounds himself with specialists smarter than him, people who help him maximize profits and minimize losses.
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. Buy low, sell high - a merchant's motto is easy to grasp.