IIRC, if it is legal to sell goods at both islands then the choice is entirely up to you. You may even sell some goods at one island and some at another, i.e. you do not have to sell all your goods at one island.
What rules "explanation" did you get that interp from? The one's I have access to don't actually state that you can sell at two islands. They only say that you must declare which island you are selling goods at. Which is ambiguous.