Wal-Mart this week brought more muscle to its competition with Amazon by buying online shopping site Jet.com. Wal-Mart Stores will pay .3 billion for the two-year-old Hoboken, N.J.-based firm. Jet.com’s “gain sharing” business model revolves around persuading customers to bunch their orders and pass on the savings in shipping costs with lower price tags. Opinions are sharply divided over the wisdom of Wal-Mart’s move. For many analysts, however, Wal-Mart’s eagerness to strengthen its online attractiveness is seen as the right driver to grow its business, even if its strategies to achieve that goal are debatable. “The Jet.com move is in that direction [of growing online sales],” said Barbara Kahn , a Wharton marketing professor and director of the school’s Jay H. Baker Retailing Center .…