for additional discussion of our principal contractual commitments, as well as our pledged assets. Purchase obligations and open purchase orders, consisting of inventory and significant non-inventory commitments, were $4.5 billion as of December 31, 2014 . Purchase obligations and open purchase orders are generally cancellable in full or in part through the contractual provisions. 

On average, our high inventory velocity means we generally collect from consumers before our payments to suppliers come due. Inventory turnover was 9 for 2014 , 2013 , and 2012 . We expect variability in inventory turnover over time since it is affected by several factors, including our product mix, the mix of sales by us and by third-party sellers, our continuing focus on in-stock inventory availability and selection of product offerings, our investment in new geographies and product lines, and the extent to which we choose to utilize third-party fulfillment providers. 

We believe that cash flows generated from operations and our cash, cash equivalents, and marketable securities balances, and borrowing available under our credit agreements will be sufficient to meet our anticipated operating cash needs for at least the next 12 months. However, any projections of future cash needs and cash flows are subject to substantial uncertainty. See Item 1A of Part I, “Risk Factors.” We continually evaluate opportunities to sell additional equity or debt securities, obtain credit facilities, obtain capital, finance, and operating lease arrangements, repurchase common stock, pay dividends, or repurchase, refinance, or otherwise restructure our debt for strategic reasons or to further strengthen our financial position. 

The sale of additional equity or convertible debt securities would likely be dilutive to our shareholders. In addition, we will, from time to time, consider the acquisition of, or investment in, complementary businesses, products, services, capital infrastructure and technologies, which might affect our liquidity requirements or cause us to secure additional financing, or issue additional equity or debt securities. There can be no assurance that additional lines-of-credit or financing instruments will be available in amounts or on terms acceptable to us, if at all. 

Results of Operations 

We have organized our operations into two segments: North America and International. We present our segment information along the same lines that our Chief Executive Officer reviews our operating results in assessing performance and allocating resources. 

Net Sales 

Net sales include product and service sales. Product sales represent revenue from the sale of products and related shipping fees and digital media content where we record revenue gross. Service sales represent third-party seller fees earned (including commissions) and related shipping fees, digital content subscriptions, and non-retail activities such as AWS, advertising services, and our co-branded credit card agreements. 

Sales increased 20% , 22% , and 27% in 2014 , 2013 , and 2012 , compared to the comparable prior year periods. Changes in foreign currency exchange rates impacted net sales by $(636)