Amazon's 2021 Prime Day sale generated "the two biggest days ever" in sales for its third-party sellers, with more than 250 million products purchased by Prime members worldwide, according to the company.

Amazon didn't disclose specific sales data, however, which it did in 2020 when the company announced that Prime Day 2020 brought in $3.5 billion for third-party sellers. This year's best-selling categories included tools, beauty, and nutrition. Total e-commerce sales in the U.S. during the Prime Day period exceeded $11 billion, 6.1 percent higher than 2020, according to Adobe Analytics' Adobe Digital Economy Index. 

Doug Gonterman, co-founder and CEO of baby spoon and bowl maker NumNum, says his Prime Day sales rose 30 percent year-over-year. Gonterman added that this year's early summer timing benefited his Pensacola, Florida-based business, as he's found that summer is usually a low sales period in the baby category. Gonterman expects the Prime Day "halo effect to linger for weeks."

Hector Gutierrez, co-founder and CEO of plant milk concentrate maker JOI, ran a 15 percent off pre-Prime Day discount and a 25 percent Prime Day discount on Monday and Tuesday. The Miami-based company saw a 799 percent increase in revenue compared with its daily average in the month of May. "The revenue we incurred as a result of the discounted price more than offset our ad spend," Gutierrez said. 

Not all small-business owners see Prime Day as a boon for business, however. Meaghan Thomas, co-founder of Pinch Spice Market, says this is the last year her Louisville-based spice company will be participating in the annual sale because she can no longer justify the ad spend. She has run Prime Day campaigns for her business since 2019, spending thousands of dollars on ads and marketing with diminishing returns each year, she says.

"We really start to question whether we have to pay-to-play on Amazon," Thomas says. "When it comes to where my advertising dollars go, the numbers don't add up for me." Spending advertising dollars on other platforms, such as social media and email campaigns, has proved to be a more cost-effective way of driving sales, she says.

Parker Russell, the founder of Bangor, Maine-based Black Ink Coffee Company, says his annual Prime Day experience since 2019 has been "uneventful," since his listings are often buried on Amazon's competitive grocery pages. However, an increase in online search traffic from consumers hunting for deals around Prime Day led to a 10x increase in Black Ink's affiliate sales on Monday and Tuesday compared with an average day. Russell advises other business owners to make deals with affiliates before Prime Day to give out exclusive coupon codes to their followers or offer higher payouts to the affiliates per transaction.

"Or you can simply pay to be the number one product that they recommend," he says.