The justices left in place an April ruling by the Wisconsin Supreme Court that found that a federal law that shields website operators from liability for user content applied to Armslist LLC, the operator of Armslist.com.
https://www.yahoo.com/news/u-supreme-court-turns-away-150459080.html
https://www.yahoo.com/news/u-supreme-court-turns-away-150459080.html