YES

TRY TO SHOP for toothpaste on Amazon and dozens of brands pop up with identical 4.5-star ratings. Crest? Colgate? Tom’s? Does it matter? Even if you try to differentiate based on reviews, you face thousands to comb through—and odds are a good number of those are fake.

Chris McCabe, who evaluated seller performance while employed at Amazon and who now runs a consulting firm for its retailers, estimates the number of inauthentic reviews on the site to be around 30%. Amazon estimates it at less than 1% and said it spent more...