f they're arguing over lucrative ink importation rights it means they've already figured out arbitrage

Not really. All sorts of arguments and fights over importation rights occurred even during the height of merchantilism. That importing goods can be profitable is a much more obvious claim than that moving goods between markets can be profitable. The second is more abstract. Moreover, they don't think of the Muggle world as that important, so the fact that the Muggle world has imbalanced prices may not be obvious to them as something to even think about.