In its escalating fight against fake product reviews, Amazon has for the first time started suing its own sellers instead of just going after other websites that promise to write five-star recommendations for cash. It's another sign that the internet giant is increasingly serious about eliminating incentives for fake reviews that erode trust, even if it means targeting the third-party retailers that hawk their wares through Amazon's platform and who, unlike the fake-review sites, are a source of revenue.