Quietly, eBay has dumped Google AdWords sponsored text ads from mobile devices in favor of Bing Ads. The move was noticed by RKG, the agency’s Director of Research, Mark Ballard, wrote about the change today.

EBay serves syndicated text ads at the bottom of category search pages as a way to further monetize its content, just like Amazon (more on that below) and thousands of retail and publisher sites. For years, eBay has served ads from Google exclusively. Now, on Ebay’s mobile site, these ads are served by Bing Ads.

As Ballard notes, this move comes on the heels of eBay acknowledging to investors that a Google SEO penalty cost the company as much as $200 million in revenue. “The Google penalty may have also prompted eBay to reconsider its status as a Google search partner,” says Ballard.

Maybe, though the fact that eBay is testing a move to Bing Ads on mobile only to start shows that the company isn’t making brash moves to get retribution against Google for the organic search penalty.

RKG found that as a result of this change, mobile accounted for 36 percent of Bing’s ad clicks in Q3, up from 25 percent in Q1. Meanwhile, Google’s mobile click share remained flat around 38 percent.

Ballard says that, typically, these sponsored listings result in high impressions but very low click-through rates. When they looked at the Bing Ads accounts of several clients, RKG found that eBay now drives 50 to 90 percent of all tablet impressions and 30 to 60 percent of smartphone impressions. Click share from eBay, though, is in the single digits. So now Bing Ads average click-through rate across RKG’s client base has dropped dramatically, down 25 to 30 percent year-over-year. Advertisers who’ve notice similar CTR declines, may also find that eBay is the culprit.

(Unlike Google AdWords, Bing Ads advertisers can analyze and exclude individual domains from the search partner network if they choose.)

How Much Can eBay & Amazon Hurt Google?

In Q3, Google’s reported click volume growth was the lowest its been in several years. Looking at the ad network performance, the picture grows bleaker. Year-over-year, paid clicks from the Google Network grew just 2 percent in Q3. Quarter-over-quarter Network click growth has been negative for two consecutive quarters. As Ballard says, “Losing eBay’s mobile search volume certainly isn’t the whole story here, but it also didn’t help.”

EBay may still just be testing Bing Ads as its mobile ad provider, but if it makes the switch permanent, and if it takes it further and adopts Bing Ads on desktop results as well, it could have a hefty impact. And that’s not Google’s only threat to its Network business. Amazon is reportedly working on a product that will compete directly with Google AdWords and will remove Google ads from its site and replace it with ads it serves itself.

Bing has had recent successes in increasing search volume through partnerships with Apple, Facebook, Twitter and Amazon. Whether or not eBay was motivated by spite to look elsewhere for an ad partner, it’s a sizable coup for Bing Ads to be considered a worthy substitute to Google.