When you shop on Amazon for a widely available product—a pair of Crocs, say, or Greenies dog treats—Amazon will pick among the merchants that offer the item and give one of them the sale when you hit “Add to Cart” or “Buy Now.”

In e-commerce, this is called winning the buy box. Amazon said its “featured merchant algorithm” picks the winner, instantly weighing available sellers’ past performance, price, delivery speed, and other factors.

Researchers at Northeastern University studying price changes on Amazon found that the merchant that won the buy box—which Amazon calls its “featured offer”—changed for seven in 10 products over a six-week period in 2016.

Five years later, we found that’s no longer the case.

When The Markup compared snapshots of 1,200 popular products 12 weeks apart, we found that the buy box was much less dynamic. The seller changed for fewer than three in 10 products in our sample.

The products we analyzed all appeared on Amazon’s first search results page of popular searches, meaning they receive prominent exposure to customers. We collected the data from an I.P. address in Washington, D.C.

Among the competing sellers for commonly available goods is Amazon itself. And when Amazon gave itself the buy box on products that other merchants also sold, it remained the buy box seller 12 weeks later for 98 percent of those products.

Overall, Amazon dominated the buy box when multiple sellers were available. We found that Amazon chose itself as the winning merchant of the “featured offer” for about 40 percent of products, while the next highest seller got the buy box in just half of one percent of popular products in our sample.

It’s hard to say why Amazon is changing the buy box winner less frequently than five years ago, said Christo Wilson, an associate computer science professor and one of the Northeastern University researchers who completed the 2016 study.

“The negative take,” he said, would be that “the market is becoming less competitive or that it’s easier for an incumbent to just sort of squat and remain stable.”

Amazon spokesperson Nell Rona declined to answer questions for this story. During congressional inquiry Amazon officials said the company doesn’t favor itself in the buy box or consider its profits in that decision.

They did acknowledge, however, that whether a product could be delivered quickly for free to Prime members is a factor in picking the seller for the buy box. Merchants typically pay extra fees for Amazon’s shipping service—Fulfillment by Amazon—to get that designation.

We found that the merchant Amazon selected for the buy box for almost every product—nine-in-10 of them—used Amazon’s shipping service. When we checked again three months later, less than 8 percent of products had changed shippers from Amazon to a third-party or vice versa.

The European Commission announced an investigation last November into whether Amazon’s criteria for the buy box results in preferential treatment for Amazon’s retail offers or sellers that use Amazon’s shipping service, which the commission said would be an abuse of Amazon’s dominant market position under E.U. antitrust rules.

In a May 2021 lawsuit, the Washington, D.C., attorney general wrote that “Amazon’s selection methods for the Buy Box winner consider factors that further reinforce Amazon’s online retail sales market dominance,” such as whether the seller uses Fulfillment by Amazon. In a court filing, Amazon responded that the lawsuit “fails to allege essential elements of an antitrust claim and, in any event, the conduct it attacks has been held by courts to be procompetitive.” The suit is ongoing.

Wilson said automated pricing algorithms may be playing a role in what The Markup found. It may also be a broader shift on the marketplace away from sellers competing to sell the same product to sellers developing their own branded products that only they are allowed to sell.

That shifts the competition away from the buy box to the search rankings, he said.

The Markup also found that Amazon gave its house brands and exclusive products a leg up in search results, above competitors with higher star ratings and more reviews, which are an indication of sales. Wilson reviewed our methodology for this investigation.

It was while testing the accuracy of findings for our main investigation that we discovered the stability of the buy box. There was a two- to four-week delay between when The Markup gathered search results and product pages. We gathered a sub-sample of listings a second time 12 weeks later to examine the effects of the delay and found they were minuscule.

“I would have thought that given that these [are] identical products and given that they are competing with similar costs, that there would be a little bit more turnover,” said Florian Ederer, an associate professor of economics at the Yale School of Management.

Shoppers can click on a link that will allow them to see more offers for a product, in addition to the one featured in the buy box. But e-commerce experts say most don’t bother: They estimate that more than 80 percent of sales on Amazon go through the buy box.

“Amazon talks about its marketplace as though it were a market,” said Stacy Mitchell, co-director of the small business advocacy group Institute for Local Self-Reliance, which has been critical of Amazon’s size and effect on retail competition in the U.S.

“This is not a market,” she added. “This is an artificial environment that Amazon controls, and it’s set up certain parameters that lead to certain outcomes.”