People around the world are lining up to buy face masks because of coronavirus concerns, but medical experts say most people who aren’t sick don’t need them. Many businesses are sold out, while others are limiting how many a customer can buy. Amazon is policing its site, trying to make sure sellers don’t gouge people. In South Korea this week, hundreds lined up to buy masks from pharmacies and discount stores. Shortages started cropping up in Asia soon after the outbreak became a crisis in China. Now many places around the world have sold out of masks. Factories in China say they are hard pressed just to meet local demand.