CNET's Declan McCullagh cites some relevant case law and federal statute:
In a 2002 decision, the Colorado Supreme Court ruled that the First Amendment protects "an individual's fundamental right to purchase books anonymously, free from governmental interference." The justices tossed out a subpoena from police to the Tattered Cover Bookstore asking for information about what books a certain customer had purchased.
And in a 2007 case, federal prosecutors tried unsuccessfully to force Amazon to identify thousands of innocent customers who bought books online, but abandoned the idea after a judge rebuked them. Judge Stephen Crocker in Wisconsin ruled that "the subpoena is troubling because it permits the government to peek into the reading habits of specific individuals without their prior knowledge or permission."
In addition, a federal law called the Video Privacy Protection Act makes it illegal for anyone selling movies to disclose customer information to anyone, including state tax collectors. The 1988 law specifically covers "prerecorded video cassette tapes," and also sweeps in "similar audio visual material" such as DVDs and Blu-ray discs [Declan McCullagh, "Amazon Fights Demand for Customer Records," CNET News, 2010.04.19].
The current Republican administration in South Dakota has often trumpted the idea of increasing the taxes you pay by applying sales tax to your online purchases. Complications like this have kept the state from making any real progress on that front.
But given our attorney general's penchant for advocating lost causes in court, perhaps he'll submit an amicus curiae brief for the North Carolina Department of Revenue.