Which game is widely credited with popularizing the 'microtransaction' business model for cosmetic items in mainstream Western gaming?

The gaming industry has transformed retail experiences through virtual economies and in-game purchases. What began as simple downloadable content has evolved into complex marketplaces generating billions in revenue. Virtual storefronts now employ sophisticated visual merchandising, psychological triggers, and retail operations that rival physical stores. Test your knowledge about a pivotal moment in gaming retail history that changed how developers monetize their products and how players interact with games.

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Team Fortress 2 with its 2010 'Mann-Conomy Update' that introduced purchasable hats and weapons
World of Warcraft with its 2010 'Celestial Steed' mount that sold for $25
Call of Duty: Modern Warfare 2 with its 2009 map pack DLC strategy
League of Legends with its 2009 launch featuring purchasable champion skins