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 merchants, these people said.

Amazon in recent weeks also has deleted thousands of suspect reviews, restricted sellers’ access to customer data on its website and stifled some techniques that trick the site into surfacing products higher in search results, according to the people.

...Sellers also are buying Amazon wholesaler accounts on the black market to gain access to volumes of product listings, people familiar with the practice said. These accounts on Amazon’s Vendor Central system are designed to enable wholesalers to edit product listings to ensure they are marketed accurately. But some sellers misuse these accounts to alter rivals’ product pages, such as by changing photos to unrelated items, these people said.

...Some Chinese firms are selling wholesale vendor accounts for roughly $15,000, or renting them for up to $1,500 a month, the people familiar with the matter said.

...Amazon has increased efforts to scrub the site of potentially suspicious reviews, according to sellers. In recent months, some product listings lost thousands of reviews in a day, a number that can take years to build. Some of the reviews were legitimate and erased inadvertently, according to the sellers and a person familiar with the matter.