Probably by putting a military blockade on the Rose Road. Keep in mind however that most of the land in the Reach is owned by the Tyrells and their bannermen, and even if the peasants of the Reach are freeholders, it’s likely that the mills are owned by the local lord, so they probably don’t have independent access to processing facilities.
Even then, transporting grain hundreds of miles to King’s Landing would mean transporting it past the Tyrell army encamped at Bitterbridge and the Tyrell scouts who were combing the north bank of the Mander. Even if you got past them, you’re bringing precious goods into a warzone – you might be able to sell your grain at a markup, but there’s no guarantee that Stannis’ army or Tywin’s army or Robb’s army or whoever don’t take your grain by force.
And even then, you’d have to take the money back through that same warzone, and even if you get home, you’re going to have to answer to your local lord about why you committed treason.
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