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.