It's not illegal for them to ask to see your receipt at the exit. It IS illegal for them to try to stop you from leaving if you decline, unless they are detain

It's not illegal for them to ask to see your receipt at the exit. It IS illegal for them to try to stop you from leaving if you decline, unless they are detaining you for suspected shoplifting (which they must specifically inform you of). And if they actually go so far as to physically prevent you from leaving? Congratulations, you just won the lawsuit lottery.