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2) No, why would I?
You can use enemy marketplaces.
Are you sure? I thought that while testing we determined that War status means you can't sue the marketplace at all.
Ask some, or one of their stewards to do it. Or make sure you have enough stockpiles. Or don't march with the army. Or don't leave anyone home to mind the store, and suffer. You have multiple options. Pick one and use it.
Maybe we can actually do both buy and sell offers automatically. But in order to make this also push trading, I would limit it to exactly one per region, so it can't be split up. If you want to do micro-management, you have to do it manually. Buy orders could be done automatically using tax gold.