My friend Geof suggests that I share this experience, as it would be good for the internet.

When I clicked on my Shopping Cart in the music store, I would get the following error:

“We could not complete your Music Store request. An unknown error ocurred (5002). There was an error in the Music Store. Please try again later.”

I emailed iTMS support about it and received the following reply:

Dear Richard,

Thank you for contacting the iTunes Music Store.

I sincerely apologize for any inconvenience this Error 5002 issue may have caused. Please note that it was necessary to clear the contents of your shopping cart to resolve the issue you have been experiencing.

It seems like one of the songs you are trying to purchase is in the middle of being modified and is unable to be purchased at this time. So this is preventing you from checking out and giving you this error code. What we recommend you doing on this particular order would be to add one
or two items time, checking the cart after each item. When the cart shows that error “5002” we have found our song that is being modified. When you find this song we would appreciate it if you would report back to us which song is giving you this error. From there we can take appropriate actions to getting this fixed.

Thank you for being an iTunes Music Store customer. We appreciate your business.

I don’t recall what was in my shopping cart, so I won’t be much help to iTMS. But maybe it’ll be help to you.