Like most states, North Carolina has faced a budget shortfall in recent years. But its response—to tax online purchases—has been more novel, and potentially lucrative, than most. The effort, which began last summer with a law requiring Amazon and other e-retailers to collect state taxes, was modeled after a similar bill in New York. Amazon sued the Empire State, however, arguing that Web sales are exempt under the protections offered to mail-order catalogs and home-shopping channels. So North Carolina shifted its approach. Earlier this year, it moved to audit Amazon in an effort to collect hundreds of millions of dollars in taxes back to 2003.