How can it be just for a merchant to sell his goods at whatever price he desires, leading to price discrepancies for the same product, and is the consumer obligated to seek the lowest price?

Religion is not a matter of opinion. Therefore, there is no limit to the permissible amount of profit, as long as the sale does not involve deceit or misleading the buyer. The original principle regarding price fixing is that it is prohibited and should only be resorted to in exceptional circumstances if people are harmed by rising prices. In such cases, it is permissible for the ruler to impose a fair price.