In 2011 a group of people in California sued Apple for holding a monopoly on selling apps for iDevices, and for selling the apps at an elevated price.
A federal appeals court revived the case in 2017 after it was originally thrown out by a federal court in Oakland, California. The Trump administration reportedly encouraged the Supreme Court to take the case.
The case swings on whether or not consumers can sue Apple over app prices, as it’s really the app developers who set the prices for the apps. It’s true that the vast majority of iDevice users go to Apple’s app store to buy apps, and that Apple takes a sizable 30% cut of each app sold there, which is surely passed on to app buyers.
If the Supreme Court upholds the decision of the appeals court and decides Apple can be sued, it could open the door for similar class action suits against companies like Amazon and eBay, as both provide dominant marketplaces where third parties sell products.