An Amazon Prime billboard in New York City. (GeekWire Photo / Taylor Soper)

Amazon said Thursday that this year’s Prime Day provided the biggest boon to third-party sellers in the sales event’s six-year history. Outside merchants sold more than $3.5 billion of goods across 19 countries, a 60% increase over last year, Amazon said in a press release. The announcement comes as Amazon faces antitrust scrutiny for its treatment of third-party retailers in the marketplace that the company operates.

Historically, Amazon has primarily used Prime Day to drive sales of the company’s products and Prime membership program.

“At the end of the day, Prime Day has always been about Amazon promoting itself first, its own products, its own memberships,” said James Thomson, a partner at the third-party seller consultancy Buy Box Experts, ahead of the event.

Related: Amazon’s first-ever fall Prime Day brings new challenges in the midst of a tumultuous year

Amazon used this year’s Prime Day to focus on the 2.3 million third-party sellers in the marketplace, including a promotion that helped drive $900 million in sales to those merchants for two weeks leading up to the event. Outside retailers are becoming increasingly important to Amazon’s business, accounting for 58% of the company’s sales in 2018. Third-party sales “again grew faster this quarter than Amazon’s first-party sales,” Amazon CEO Jeff Bezos said in the company’s second quarter earnings report this year.

The push to support third-party retailers comes amid antitrust scrutiny from lawmakers and former Amazon sellers who claim the company abuses its role as marketplace arbiter.

Last week, the U.S. House Subcommittee on Antitrust released a 451-page report on the ways in which Apple, Facebook, Google and Amazon capitalize on and allegedly abuse their market power to benefit themselves. The report dedicates an 83-page section to the Seattle-based e-commerce giant, informed by internal company emails, extensive market research, interviews with third-party retailers, submissions from industry groups, and testimony including the a hearing this summer with Bezos and others.

“Amazon’s pattern of exploiting sellers, enabled by its market dominance, raises serious competition concerns,” the report says.

Prime Day fielded plenty of criticism beyond Washington D.C., too.

The American Booksellers Association ran a “Boxed Out” campaign with anti-Amazon slogans and installations in bookstore windows and posts on social media. Independent shops across the country posted cardboard signs in storefronts with slogans like “Bookstores vs. Billionaires” and “Books curated by real people not a creepy algorithm.” Fake cardboard books were displayed with titles such as “To Kill A Locally Owned Bookstore.”

Meanwhile, Bloomberg reported that some workers were complaining that the rush to fill Prime Day orders was breaking protocols put in place to ensure warehouse worker safety during the pandemic. Employees at a Staten Island facility said the company has been pressuring employees to be more productive and warning that slowness could lead to termination.

“Amazon’s Prime Day means more injuries and unacceptable levels of stress for its workforce,” said Stuart Appelbaum, president of the Retail, Wholesale and Department Store Union, in a statement. “During normal times it’s a grueling period for workers. During a global pandemic it may well push workers beyond their limit.”

RWDSU is one of several unions working to organize Amazon and Whole Foods employees.

Senators Elizabeth Warren, Ed Markey, and Joseph Kennedy sent a letter Wednesday to Bezos expressing concern about warehouse worker injuries associated with the event.

Analysts predict this year’s Prime Day will be the biggest in Amazon’s history, with revenue of about $7.5 billion expected. Amazon says Prime members saved more than $1.4 billion in discounts this Prime Day and the most popular item purchased globally was the Echo Dot.

New York Times reporter Shira Ovide noted that Amazon did not tout Prime membership sign-ups during Prime Day this year as it has in the past.

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