I'm actually having the same problem. I can't buy any songs, and I've tried logging out/logging back into the iTMS to no avail. It errors for both free and paid songs that I want to buy. Any ideas?
-Chase
I'll post back when I find the fix, but if you go to iTMS help, it'll tell you to delete all of a certain type of cookie from them, and all will be fine.
I had the problem, I did the fix, and all is well!