That’s the new finding from a study of over a million eBay transactions across the U.S. published Friday in the journal Science Advances.

How much more? Comparing sales on brand-new items, women make on average about 80 cents for every dollar a man does.

With help from eBay Labs, sociologist Tamar Kricheli-Katz and economist Tali Regev peered into transaction data from 2009-2012, looking at which sellers were men, which were women, and how much each made selling identical items online.

Regev says what’s interesting about studying pay disparities on a website like eBay is that unlike workforce wage gap studies, the difference in pay in an online auction can’t be explained by human factors, such as experience.

“In labor markets you can always say there are differences between men and women,” Regev says. But when two people are selling the exact same brand new product on eBay? “There’s not much you can say about the differences in the product itself,” Regev adds.

The pay gap on eBay mirrors what economists know about the American labor market. The U.S. Bureau of Labor Statistics reports that women make approximately 81 cents on the dollar, a figure that’s been widening recently. For female CEOs, the gap is worse: they make about 70% compared to male chief executives.

Abby Wambach on the Gender Pay Gap: Enough Is Enough

Kricheli-Katz and Regev weren’t satisfied just seeing the eBay data though; they wanted to be sure online auctioning biases were real. So they set up their own experiment. They invited 116 people, both men and women, to bargain for a $100 gift card. Some study participants wagered on a gift card from an imaginary online vendor named “Allison,” while some placed bets on “Brad’s” $100 card. “Allison” made on average 7% less than “Brad” for the exact same gift card.

But it’s not just women who get a raw deal hawking on eBay.

In a separate study published in the Rand Journal of Economics last October, professors at Yale Law School and Harvard University found when photos of baseball cards on eBay were held in a black hand, they made 20% less than white-handed sellers.

“The way we interact in markets is affected by our beliefs about status and the social script we have in mind,” Regev says.

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Regev’s colleague Kricheli-Katz says she hopes people presented with this new data will become more aware of their own biases. “Maybe if people know that, this process will become more conscious, and over time the gap will narrow,” she says.

But sometimes, it does pay to be a woman on eBay.

Women outsold men in certain categories, including baby products and toys for pets. Women also had higher “star” ratings, indicating better feedback from buyers. And when women sold used goods, the wage gap nearly disappeared: women earned on average 97 cents on the dollar.

The researchers think there may be something about the perceived gender of a product that affects the gap. Is it possible women get more for items like handbags, but men get more for a set of golf clubs?

The answer to that question… will be sold to the highest bidder.

Update: (Feb. 19) In a statement to Fortune, eBay reported that while the company does not reveal the gender of sellers online, “48% of our top U.S. consumer sellers are female.”