"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. Congress is involved because interstate commerce typically falls under the Commerce Clause of the Constitution. Opponents of remote vendor sales and use tax collection cite the complexity of the myriad state and local sales tax systems and the difficulty vendors would have in collecting and remitting use taxes. Proponents would like Congress to change the law and allow states to require out-of-state vendors without nexus to collect state use taxes. These proponents acknowledge that simplification and harmonization of state tax systems are likely prerequisites for Congress to consider approval of increased collection authority for states."
CRS Report for Congress, R41853