Women in the United States are paid only 79 cents on the dollar compared with men doing the same job. But at least gender melts away in the digital economy of the Internet, right? Nope. A study of more than 1 million auctions on the online commerce site eBay finds that women receive consistently less money than men for selling the very same products.

Getting the data to prove this gap is no easy task. Luckily eBay, which is based in San Jose, California, opened its doors to researchers 2 years ago. Among the first to gain access were two from Israel—Tamar Kricheli-Katz, a sociologist and legal scholar from Tel Aviv University, and Tali Regev, an economist from the Interdisciplinary Center Herzliya—who together study inequality. They wanted to know whether the perceived value of a product depends on the gender of the seller. eBay represented the perfect natural experiment: Millions of identical products are sold by men and women through auctions where the buyers determine the final price.

But is the gender of a seller even apparent to buyers on eBay? Sellers can declare personal information when they register, including gender, so Kricheli-Katz and Regev knew who identified as male or female in their data. To see whether gender was apparent in an eBay auction, they challenged 400 people to guess the gender of 100 randomly chosen sellers. Just using clues like the names of the sellers and what other items they tended to sell, participants correctly guessed the gender of 56%, declared 35% unguessable, and got less than 9% wrong. So gender can come through if buyers are paying attention. But does that matter to the final sale price?

Kricheli-Katz and Regev narrowed their focus to the 420 most popular products auctioned on eBay between 2009 and 2012, amounting to 1.1 million transactions. For example, thousands of iPods, both used and brand new in the original wrapping, were auctioned off by men and women. If the men got more money from the same product, then that could stem from an unconscious gender bias on the part of the buyers.

Researchers found that when the seller of these popular items was self-identified as female, the auction got fewer bids and a lower final price. For used items, the gender gap was small, with female sellers getting 3% less money on average. But for new products, women received only 80 cents for every dollar that men got for auctioning a similar product on eBay, the team reports today in Science Advances. The reputation scores of the sellers could not account for that difference, nor could any of the auction options such as the "buy it now" or initial prices. The most striking example was gift cards—vouchers for a fixed amount of money that can be spent at certain stores—for which the gender gap persisted, even though the value is obviously the same no matter the seller.

One possible explanation for the smaller gap for used products, say the researchers, is that potential buyers—subconsciously or not—trust women to describe the quality of used products more accurately than men. But what about the much larger gap for new products?

Kricheli-Katz and Regev suspected that it might come down to the way men and women describe their wares. Perhaps men used more flattering terms on the whole than women. So they performed a computer analysis on the titles and subtitles of the advertisements, scoring them for words that reflected positive or negative sentiment. "The sentiment analysis showed that women and men sellers do, indeed, resort to different sentiments," Kricheli-Katz says, but "the difference we found is relatively small." Controlling for sentiment, there was still a gender gap of 3% for used products and 19% for new.

The study is "a new addition to the growing body of evidence that gender inequality doesn't end when people go online," says Benjamin Mako Hill, an Internet and communication scholar at the University of Washington, Seattle. "The fact that gender seems to lead to such a gap in eBay, where gender is such a relatively tiny signal, is striking." And how to close the gap? "One approach that is sometimes suggested is making gender less visible in exactly the way that eBay does," he says. "These results provide little support for the idea that type of approach will be effective in practice."