II, “Financial Statements and Supplementary Data—Note 1—Description of Business and Accounting Policies”). Increased unit sales were driven largely by our continued efforts to reduce prices for our customers, including from our free shipping offers and Amazon Prime, by increased in-stock inventory availability and increased selection of product offerings, and by the impact of Zappos, which we acquired in the fourth quarter of 2009. 

The International sales growth rate was 33%, 31%, and 33% in 2010, 2009, and 2008. The sales growth in each year primarily reflects increased unit sales and a larger base of sales in faster growing categories such as electronics and other general merchandise. Increased unit sales were driven largely by our continued efforts to reduce prices for our customers, including from our free shipping offers and Amazon Prime, and by increased in-stock inventory availability and increased selection of product offerings. Additionally, changes in currency exchange rates positively (negatively) affected International net sales by $(107) million, $(174) million, and $131 million in 2010, 2009, and 2008. We expect that, over time, our International segment will represent 50% or more of our consolidated net sales. 

Sales of products by marketplace sellers on our websites represented 31%, 30%, and 28% of unit sales in 2010, 2009, and 2008. Revenues from these sales are recorded as a net amount. Because we focus on operating profits, we are largely neutral on whether an item is sold by us or by another seller. 

Supplemental Information 

We expect our net cost of shipping to continue to increase to the extent our customers accept and use our free-shipping offers at an increasing rate, including through membership in Amazon Prime; to the extent our product mix shifts to the electronics and other general merchandise category; to the extent we reduce shipping rates; and to the extent we use more expensive shipping methods. We seek to mitigate costs of shipping over time in part through achieving higher sales volumes, negotiating better terms with our suppliers, and achieving better operating efficiencies. We believe that offering low prices to our customers is fundamental to our future success. One way we offer lower prices is through free-shipping offers, as well as through membership in Amazon Prime. 

Operating Expenses 

Operating expenses without stock-based compensation are non-GAAP financial measures. See “Non-GAAP Financial Measures” and Item 8 of Part I, “Financial Statements and Supplementary Data—Note 1—Description of Business and Accounting Policies—Stock-Based Compensation.” 

Cost of Sales 

Cost of sales consists of the purchase price of consumer products and content where we are the seller of record, inbound and outbound shipping charges, and packaging supplies. Shipping charges to receive products from our suppliers are included in our inventory, and recognized as cost of sales upon sale of products to our customers. 

The increase in cost of sales in absolute dollars in 2010 and 2009 compared to the comparable prior year periods is primarily due to increased product and shipping costs resulting from increased