Retailers will be closely watching the outcome of the U.S. Supreme Court’s decision to revisit a 26-year-old case which has limited states’ taxing authority over online sales.

The Supreme Court, heeding calls from traditional retailers and dozens of states, has granted review of South Dakota v. Wayfair, Inc., in which retailers challenge the 1992 ruling in Quill Corp v. North Dakota as obsolete. Quill held – in a pre-internet era – that states cannot impose sales and use tax collection obligations on retailers without a physical presence within a state. As a result, online retailers without a physical presence in a state have developed a pricing advantage over retailers located within the state.

In the meantime, Congress is considering a number of legislative proposals addressing state taxation of online sales, including the Remote Transactions Parity Act, which would allow states to require out-of-state sellers to collect sales tax. The bill has the support of the National Retail Federation.

Additionally, with many major online retailers already collecting and remitting sales and use taxes to states, some states are turning their legislative sights on smaller third-party sellers that move their merchandise over marketplace platforms hosted by e-commerce giants. Last year, Minnesota, Washington, Pennsylvania, and Rhode Island all enacted “marketplace provider laws”—first-ever statutes imposing tax collection duties on marketplaces for sales by their third-party sellers.

Estimates of uncollected state taxes on such transactions vary, but many analysts put the number at several billion dollars annually from Amazon alone. An analyst with Internet Retailer, an e-commerce news and analytics publication, recently estimated the state tax gap could be as high as $3 billion per year from Amazon sellers, and $4 billion when sellers on smaller platforms are added.