Four retailers, each with their own challenges, are particularly important to watch in 2013 as they attempt to navigate modern economic challenges, write Ann Zimmerman and Dana Mattioli. Best Buy needs to attract more online sales of its own to combat against "showrooming" at their bricks-and-mortar stores. J.C. Penney needs to shake off a fusty image and prove that its makeover made it more attractive. Sears has been selling itself off in pieces for liquidity, but what remains is not doing well. And RadioShack needs leadership to direct it after a shift to selling mobile devices has largely failed.