This Congressional Research Service (CRS) report, prepared for Congress and made available online by the Federation of American Scientists (FAS), "The United States Bureau of the Census estimated that $4.1 trillion worth of retail and wholesale transactions were conducted over the Internet in 2010. That amount was 16.1% of all U.S. shipments and sales in that year. Other estimates, based on different data, projected the 2011 so called e-commerce volume at approximately $3.9 trillion. The volume, roughly $4 trillion, of ecommerce is expected to increase, and state and local governments are concerned because collection of sales taxes on these transactions is difficult to enforce. Under current law, states cannot reach beyond their borders and compel out-of-state Internet vendors (those without nexus in the buyer's state) to collect the use tax owed by state residents and businesses. The Supreme Court ruled in 1967 that requiring remote vendors to collect the use tax would pose an undue burden on interstate commerce. Estimates put this lost state tax revenue at approximately $11.4 billion in 2012."
