Also keep in mind that in our world, we have global transportation that can get goods from one side of the world to the other in days. Using sailing ships for that purpose, you get half that distance in months. Transportation costs would drive up the price considerably, for any good not produced locally. Also, a more practical reason for not overstocking the ship with skooma, even if you could afford it, would be the logistics of getting past port authorities. If every crate you have has skooma, then it would be amazing if it wasn't discovered.