The company started as an online book seller, then quickly expanded into music and movies, and finally into electronics and home improvement products. Another part of Amazon's retail strategy is to act as a channel for other retailers to sell their products and take a percentage cut of each purchase. Amazon is not required to maintain inventory of slower-selling products. This strategy has made Amazon a leading "long tail" retailer, expanding its available selection without a corresponding increase in overhead costs. Incorporating this "long tail" retail model, Amazon introduced the sale of used products through its seller marketplace. This retail model provides another retail revenue stream for the company without the need to stock products in its own warehouses. Shipping and advertising are handled solely by the sellers, with Amazon taking a percentage cut of each sale simply by providing the channel. Internet Services On the consumer side, Amazon provides services like Amazon Prime, which offers free two-day shipping on retail purchases, on-demand video streaming, and free access to the Kindle library, all for a year