MarketService is a Potentially Unwanted Program (PUP) running on Mac devices. It is an intrusive piece of software, which may introduce annoying changes concerning your favorite Web browser. The purpose of MarketService is to modify the settings of Safari, Google Chrome, Mozilla Firefox, and other browsers. These changes are not dangerous or malicious, but they may end up hindering the user's Web browsing experience.
Typically, Mac PUPs like MarketService are used to promote low-quality search engine by configuring them as the default new tab page or search service that the browser uses. Because of this, users may experience regular redirects to unreliable search results, often mixed with ads, pop-ups, and alerts.
The best way to keep intrusive apps like MarketService away from your Mac is to take preventive measures by securing your device with a reputable security software suite. Furthermore, it is recommended to be careful with the types of files you download from the Internet – stick to reputable sources of installers/updates, and avoid browsing torrent trackers or unreliable pages.