Why was there a market price within the camp? Who would buy when abundant food is Hefker in front of him?! This is like Malbim writes (verse 17), that the king owned the spoils; his Shali'ach sold to people. Alternatively, most people were weak due to the hunger. It would take them a long time to bring loot to their houses and return to the camp to take more. Some entrepreneurs brought wagons (horses were now available to lead them) that can carry many (e.g. 50) times as much as a person. They bought food from looters, to quickly load the wagons with food, bring the food to their homes, and return to buy (what looters took meanwhile) and load them again. The sellers profited from this, and also the buyers expected to profit from this. Later, some virtuous entrepreneurs decided not to profit, and resold the grain for the same price they paid for it; also the greedy entrepreneurs were not able to charge more. Or, after seeing the miracle, all realized that Devar Hashem will be fulfilled, so no one would pay more than the prophesized price. (PF)