What innovative retail strategy did Amazon pioneer that fundamentally transformed e-commerce economics and became known as the 'flywheel effect'?

Amazon has become one of the world's most valuable companies through strategic innovation and retail transformation. While many focus on their e-commerce platform and Prime membership, there's a specific business strategy that Jeff Bezos implemented early on that fundamentally changed retail economics and allowed Amazon to scale beyond competitors. Test your knowledge of this pivotal business strategy that revolutionized online retail.

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The long tail strategy - focusing on selling small volumes of hard-to-find items to many customers
The marketplace model - allowing third-party sellers on their platform to dramatically expand selection
The subscription model - creating Amazon Prime to generate recurring revenue and customer loyalty
The loss leader strategy - selling popular products below cost to attract customers to higher-margin items