Booksfree, which makes its home in a 3,000-square-foot Vienna warehouse next to a pizza-delivery joint and several industrial outfits, is run by unlikely entrepreneurs in a hostile economy. Its founders, W. Douglas Ross and Andrew E. Bilinski, at 60 and 54, respectively, are a generation older than the twentysomethings who rode the Internet boom to its peak three years ago. Booksfree, which came late to the online commerce scene, is one of the few remaining survivors.