Comment 2 for bug 1438999

Did you log into your Ubuntu One account via System Settings, before going to the Ubuntu Store scope to re-install your apps?

There is no way to know if you have purchased any apps, prior to logging in to your account. If no account exists on the phone, then we must show the buy button, rather than the install button. However, 'buy' will simply require you to log in, and then will immediately result in the app being installed, rather than going through the buy process.