Unloading 500t of cargo should take a while and should not be instant. Maybe later when it is possible to walk around one can see how his ship gets unloaded or loaded 
That would add to the level of immersion. If the traders are concerned you could increase trade profit a little to compensate for the lost time.
If you add the option to pay the workers to unload/load faster the player has one decision more he can make.
That would add to the level of immersion. If the traders are concerned you could increase trade profit a little to compensate for the lost time.
If you add the option to pay the workers to unload/load faster the player has one decision more he can make.