Dwilight is broken at the moment or I would look up exactly what he said. I get a report when he sells the food that says he collects the money from the sale. As my character is in need of money, I complained about this, he told me that gold is removed from him when he buys it, so he's not making a profit.
What most probably happens is that the banker is responsible both for the selling and the buying. The report says correctly that he pockets money from the sale, but it is his own money, so he's not really making a profit. He would only make money if was selling food to a pre-established buying offer that was set up by someone else.
You can avoid this by removing his access to your granaries, but you'll have to manage your food yourself then.
