Amazon.com Inc. is fighting a barrage of seller scams on its website, including firing several employees suspected of having helped supply independent merchants with inside information, according to people familiar with the company’s effort.

Amazon was investigating suspected data leaks and bribes of its employees, The Wall Street Journal reported in September. Since then, the company has dismissed several workers in the U.S. and India for allegedly inappropriately accessing internal data that was being misused by disreputable...