Have you encountered itunes error 5002 when purchasing a song or attempting to access your cart? The exact message is "Could not purchase 'Name of the song I was downloading.' An unknown error occured (5002)." Here are some ways to fix itunes error 5002, directly from the Apple site.
If you've got apps in your library, delete any that have just a black icon. This should solve the 5002 error and then you can download the deleted apps again for free.
Also, do a re-installation with Pacifist 2.5.2. As a result, you can see your iTunes cart with my all your selections still in it. However, you cannot add anything to the cart. So make a snapshot of your cart for recreation later and delete everything in it. After so doing you can now add items to the cart and all functionality will return!