In which year did the first major commercial implementation of microtransactions (in-game purchases) appear in a mainstream video game?

The gaming industry has revolutionized retail with innovative monetization strategies. One of the most transformative retail innovations in gaming history was the introduction of microtransactions and in-game purchases. This poll tests your knowledge about when this significant retail trend began changing the gaming landscape forever, setting the foundation for today's virtual economies worth billions of dollars.

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2006, when Bethesda released horse armor DLC for The Elder Scrolls IV: Oblivion
2009, when Zynga introduced virtual goods purchases in FarmVille on Facebook
2011, when Team Fortress 2 transitioned to a free-to-play model with cosmetic microtransactions
2003, when Second Life launched with its virtual marketplace and Linden Dollar currency