Which happens how often exactly? And if the seller is not required to display the price, knowing that he has every reason to charge as high a price as he can get away with, what is a reason in the vast majority of transactions that he might be inclined to charge any less than that maximum?

Your entire argument is based on an edge case. And if regulations have to be created, I'm calling for the European model. No questions, no conditions, you are listing the price.