In tens of millions of reviews on Web sites like Amazon.com, Citysearch, TripAdvisor and Yelp, new books are better than Tolstoy, restaurants are undiscovered gems and hotels surpass the Ritz. Or so the reviewers say. As online retailers increasingly depend on reviews as a sales tool, an industry of fibbers and promoters has sprung up to buy and sell raves for a pittance. “For $5, I will submit tw