We have no idea why (sorry!), but there’s apparently a surge in “missing” apps in the App Store tonight. This results in the error message “The item you tried to buy is no longer available” when you attempt to purchase.
Based on our search traffic hitting this post, the problem started earlier this evening and is affecting at least the US, UK, Canadian and Australian app stores. Apple’s Pages is reportedly one of the affected apps.
Chances are this is a transient glitch and will be sorted out soon, so the best advice for the moment is to be patient and try again in an hour or so. If it’s not cleared up by the AM hours on the East Coast we’ll update this post.