Racism affects online sales across much of the nation, suggests a study.

In the Stanford Institute for Economic Policy Research effort by economists Jennifer Doleac and Luke Stein, at least least three online ads for a used Apple iPod were placed in 300 regions nationwide from 2009 to 2010. "Each ad contained a photo of the iPod held by a black hand, a white hand, or a white hand with a wrist tattoo," says the study.

The economists ran each ad for 12 hours, offering to sell the iPod to the highest bidder.What they found:

• Black sellers received 13 percent fewer responses and 17 percent fewer offers than white sellers. • The average offer received by black sellers was 2 percent to 4 percent lower, despite the self-selected—and presumably less biased—pool of bidders responding to these ads. • The effects were similar for tattooed sellers, suggesting a role for statistical discrimination. • Buyers corresponding with black sellers exhibited lower trust: They were 17 percent less likely to include their name in e-mails, 44 percent less likely to accept delivery by mail, and 56 percent more likely to express concern about making a long-distance payment.

"Clearly, black sellers are at a significant disadvantage to whites," say the authors, particularly in places where few iPods went on sale, where black sellers received 24% fewer offers. In markets where 20 or more iPods went on sale a week, there was little discrimination.

Only in Western states did buyers not make fewer offers to black sellers. In the Northeast, they received 32% less; in the Midwest, 23% fewer; and in the South, 15% fewer. Black sellers also received fewer offers in high-crime and racially segregated regions.

"Black sellers are at a significant disadvantage on average, but their outcomes depend greatly on various features of their local markets, including the level of competition and the degree to which local buyers are wary," concludes the study.

By Dan Vergano