tool and intend to continue offering them indefinitely. 

Sales of products by marketplace sellers on our websites represented 29%, 28%, and 28% of unit sales in 2008, 2007, and 2006. Since revenues from these sales are recorded as a net amount, they generally result in lower revenues but higher gross margin per unit. Since we focus on profit dollars rather than margins, we are largely neutral on whether an item is sold by us or by another seller. 

Gross profit growth is also affected by changes in exchange rates—see “Effect of Exchange Rates” below. 

North America segment gross margins in 2008 decreased by 69 basis points compared to 2007 resulting primarily from our efforts to continue reducing prices for our customers, including from our free shipping offers and Amazon Prime, and a larger percent of overall sales in lower margin categories such as electronics and other general merchandise, partially offset by increases in other revenue and improvements in vendor pricing. 

North America segment gross margins in 2007 decreased by 90 basis points compared to 2006 resulting primarily from our efforts to continue reducing prices for our customers, including from our free shipping offers and Amazon Prime, and a larger percent of overall sales in lower margin categories such as electronics and other general merchandise. 

International segment gross margins in 2008 increased by 25 basis points compared to 2007 resulting primarily from increases in sales of products by other sellers and improvements in vendor pricing, partially offset by our efforts to continue reducing prices for our customers, including from our free shipping offers and Amazon Prime, and a larger percent of overall sales in lower margin categories such as electronics and other general merchandise. 

International segment gross margins in 2007 increased by 38 basis points compared to 2006 resulting primarily from increases in sales of products by other sellers, offset partially by our efforts to continue reducing prices for our customers, including from our free shipping offers and Amazon Prime, and a larger percent of overall sales in lower margin categories such as electronics and other general merchandise. 

Supplemental Information 

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 that result in a net cost to us in delivering products, as well as through membership in Amazon Prime. To the extent our customers accept and use our free shipping offers at an increasing rate, including memberships in Amazon Prime, our net cost of shipping will increase. We seek to partially mitigate the costs of lowering prices over time through achieving higher sales volumes, negotiating better terms with our suppliers, and achieving better operating efficiencies. 

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.” 

Fulfillment 

The increase in fulfillment costs in