"Imagine if Wal-Mart tried to offer one-half of its customers one price and the other half a different price. Or even if they tried to change the price for a week to see what would happen."

Someone from the first group would set up a retail store and outcompete Walmart for the second group. Just like what would happen (but much more efficiently since there's no cost inherent in moving stuff around) in a virtual world that allows trade.