Have the GM pick

Use this option sparingly. Reserve it for moves where you're in the best position to choose, based on your agenda, principles, prep, and knowledge of the situation—or when you want to emphasize a PC's lack of control.

For example, because Defy Danger can apply to so many situations, it makes sense for you, the GM, to decide whether a 7-9 leads to a lesser success, a cost, or a consequence, and what exactly that means. And because Trade & Barter (page 540) involves deciding what is or is not available—something entirely outside the PCs' control—it makes sense for the GM to pick between the 7-9 results.