> What would prevent someone from, say, buying up all the water and then not selling any of it, or selling just a little to a handful of rich people?
First, no one has enough money to buy up all of the water. Second, even if they did, it wouldn't make sense not to sell it. Most people like making a profit and having more money. It wouldn't make sense to only sell it to wealthy people, because they don't use enough more water to exhaust supply in most places people inhabit and they'd be forgoing a lot of profit from selling water to normal people. Also governments also often sell water to politically connected business and agriculture groups at a lower rate than they sell to normal people.
First, no one has enough money to buy up all of the water. Second, even if they did, it wouldn't make sense not to sell it. Most people like making a profit and having more money. It wouldn't make sense to only sell it to wealthy people, because they don't use enough more water to exhaust supply in most places people inhabit and they'd be forgoing a lot of profit from selling water to normal people. Also governments also often sell water to politically connected business and agriculture groups at a lower rate than they sell to normal people.