million , and $169 million for 2015 , 2014 , and 2013 . As of December 31, 2015 , our federal net operating loss carryforward was approximately $1.1 billion and we had approximately $622 million of federal tax credits potentially available to offset future tax liabilities. Our federal tax credits are primarily related to the U.S. federal research and development credit, which was made permanent in 2015. As we utilize our federal net operating losses and tax credits, we expect cash paid for taxes to significantly increase. We endeavor to manage our global taxes on a cash basis, rather than on a financial reporting basis. 

Our liquidity is also affected by restricted cash balances that are pledged as collateral for standby and trade letters of credit, guarantees, debt, and real estate leases. To the extent we process payments for third-party sellers or offer certain types of stored value to our customers, some jurisdictions may restrict our use of those funds. These restrictions would result in the reclassification of a portion of our cash and cash equivalents from “Cash and cash equivalents” to restricted cash, which is classified within “Accounts receivable, net and other” on our consolidated balance sheets. As of December 31, 2015 and 2014 , restricted cash, cash equivalents, and marketable securities were $285 million and $450 million . See Item 8 of Part II, “Financial Statements and Supplementary Data—Note 7— Commitments and Contingencies” 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 $6.2 billion as of December 31, 2015 . 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 8 for 2015 and 9 for 2014 and 2013 . 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, as well as 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.