Sandell Asset Management Corp., the sometimes-activist hedge fund, is urging Barnes & Noble Inc. to explore a sale amid a hostile environment for retailers. The New York-based hedge fund said it has built a “meaningful position” in Barnes & Noble, in a letter to the bookseller’s board on Tuesday. Sandell said the company should explore a sale, which could fetch more than $12 a share, or a 69 percent premium over where shares closed on Monday. “Stakeholders would be better served if Barnes & Noble were operated as a private company or as a division within a larger company, which...