If your economy adapts to seasons and trade then you would have to remember: 15* 4 * amount of conditions* trade points, to understand what can trade where.
I can't help but wonder if the core problem here isn't the number of trade items but a poor user interface that expects you to remember what trades for what at these points. If mapping and writing down dialog clues is out of fashion in modern games, why should the player be expected to memorize trading good locations and differentials across an entire map?
I'm actually surprised at the number of trading games that do this as it creates a somewhat negative experience of having to know the game environment before you play. At the very least the UI should highlight what you've traded where as ports are visited, and for a less hardcore / micromanagement game I don't see a problem of just showing differentials everywhere (Endless Sky, a sci-fi trading game, does once you've visited or bought maps and it makes trade a fun diversion even if it does oversimplify the gameplay bit).