In the 12 years since eBay's launch, the online auction house has established itself as a one-stop shop for all things rare, kitschy, and collectible. But recently, a small group of economists have mined the site for a different prize: clues on how people spend their money.

Behind the millions of online auctions lies a virtual mini-economy flush with raw data. Harvesting this information has fed a new branch of economics, one that has proved again and again that shoppers act in unexpected ways.

Auctions can be hard to predict. Various items, be they ancient coins or next-generation electronics, can inspire odd behavior in buyers.

Late last year, crowds waited for days outside retail stores to buy the new Play­Station 3 video-game system. Many of those in line then sold the $600 machines on eBay for thousands of dollars.

But when Apple's iPhone drew similar crowds last month, resale prices on eBay were rather flat.

This unpredictability makes research difficult. After all, how do you quantify fashion? Or translate passion into a line chart?

Hoping to squelch this X-factor, Ulrike Malmendier, an economist at the University of California at Berkeley, tracked auctions of common items – things readily available online or in stores but offered on eBay at a discount.

Most economists assume these kinds of auctions are largely immune to the passions and unpredictabilities of ravenous bidders, she says. Simple bargain hunting, they hope, would bring out our inner homo economicus, someone who acts in their self-interest to get the best deal possible.

No such luck, she says.

Ms. Malmendier tracked 166 auctions offering "CashFlow 101," a personal-finance-themed board game. During the seven-month trial, the game's designer sold the box set on his website for $195.

Meanwhile, eBay sellers usually offered an opening price of about $45 and set a one-click, "buy it now" price of about $125. It looked like a great deal for buyers. They could pay less than retail to end the auction immediately or place bids in the hope of fetching an even lower price.

But this is where eBay users fell prey to what Malmendier and her coauthor, Stanford University economist Hanh Lee, call "bidder's curse." Apparently, some bidders grew so enthusiastic about winning the auction that they lost sight of the "buy it now" price, sometimes offering more than $185.

"We found that in 43 percent of the auctions the bidders ended up paying more than the 'buy it now' price," Malmendier says.

"This is really huge. It's far more than I could have expected."

Confused, the team tried a larger sample – this time observing thousands of iPod auctions. In that case, 45 to 50 percent of eBay auctions exceeded the "buy it now" price, she says. Expanding the pool again, they found that the quirk affects expensive and cheap items, men's cologne and women's perfume, and books by liberal Sen. Barack Obama and by conservative commentator Bill O'Reilly. The Romans had a term for this auction-house "curse," Malmendier says, "They called it calor licitantis – bidder's heat."

Before eBay, economists had few ways to test their theories about auctions. They could recruit and test volunteers, but that often meant subjects already knew they were being watched. Some experiments in the mid-1990s involved digital auctions on primitive Internet message boards. But the pool of potential bidders was too small back then to draw any broad economic conclusions, says Tanjim Hossain, an assistant professor at the Hong Kong University of Science and Technology.

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