A ruling favoring South Dakota could eventually lead to online customers paying more for many purchases.
Such a ruling could help small brick-and-mortar retailers compete with online rivals while delivering up to $18 billion into the coffers of the affected states, according to a 2017 federal report. The justices are due to decide the case by the end of June.
South Dakota depends more than most states on sales taxes because it is one of nine that do not have a state income tax. South Dakota projects its revenue losses because of online sales that do not collect state taxes at around $50 million annually, while its opponents in the case estimate it as less than half that figure.
The justices heard the case against a backdrop of Trump's harsh criticism of Amazon.com, the dominant player in online retail, on the issue of taxes and other matters. Trump has assailed Amazon CEO Jeff Bezos, who owns the Washington Post, a newspaper that the Republican president also has disparaged.
Amazon, which is not involved in the Supreme Court case, collects sales taxes on direct purchases on its site but does not collect taxes for items sold on its platform by third-party venders, amounting to about half of total sales.
South Dakota is supported by industry groups representing major retailers that have brick-and-mortar stores, and therefore already collect state sales taxes. The National Retail Federation, which supports the state, has a membership that includes Walmart Inc and Target Corp, as well as Amazon.
E-commerce companies supporting Wayfair, Overstock and Newegg include two that provide online platforms for individuals to sell online: eBay and Etsy.
The 2016 South Dakota law requires out-of-state online retailers to collect sales tax if they clear $100,000 in sales or 200 separate transactions. The state sued a group of online retailers to force them to collect the state sales taxes, with the aim of overturning the 1992 precedent.