We are aware of the paypal issues that people have been having. Currently if you attempt to purchase the game it will give you an error about requesting an outdated version of paypal. We are in contact with Paypal attempting to resolve the situation.
For now, if you simply refresh the page that you end up on("You have requested an outdated version"), it should take you past that point, allowing you to buy the game.
Sorry for any inconvenience. Most people on the forums probably already have the game, but if any friends ask, this is the workaround. We're trying to get the real issue fixed now.