Divide the class into shopkeepers and shoppers. (You decide the balance.) Give shopkeepers a kind of shop (e.g. fishmonger) and a list of things to sell. They make up their own prices. Give shoppers a list of things to buy (different lists for each group). Each group tries to buy their things at the lowest price. Some stores (such as grocery stores and convenience stores) will sell some of the same things, so they may start discounting, etc. Make sure they all use English at least for the prices.