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If I'm understanding you correctly, I believe that the money comes from the Banker...and goes to the Banker. So only the food gets moved in cases like that.
Which is what happens as I have done so more than once myself.
How do you do that? I can only cancel the offers I made myself, never accept them. The checkbox does not appear.
No checkboxes, they are for brokering. You need to enter the market you wish to buy for, through Command/Regional food status, then just find the offer you want and press "trade". But you need to enter through specific matket.