If profit is your focus, then don't hand control over to the banker.
If you don't want to deal with it, don't want to screw over your realm, and or don't care about profit, then you can let a steward do it, or let the banker do it. Or you can set up an auto sell order, and let it happen all by itself, but share the profit with your knights.
Or you can allow the banker to do the sales, and then demand that he send half the profits to you.