The Buy button is not working because the application has been discontinued. I've tried to find a solution to this problem, but it doesn't appear to exist one. I've also installed the application only to find out that indeed the Buy button doesn't work at all making it impossible to reach the buy page for the application. Sometimes you can use the trial version for an unlimited time. You need to remove it and then install it back.
Remove the application using Revo Uninstaller. It will force it to uninstall by wiping out all traces of its presence from the system making it suitable to install it again.