Five years ago, eBay ditched Yahoo’s ads in favour of Google’s. And now, eBay drops Google AdWords’ sponsored text ads from mobile devices in favour of Bing Ads.

The hushed transition was noticed by Mark Ballard, director of research at RKG, who wrote about it in his blog post. He pointed to the manual action penalty eBay received from Google as the impetus behind the drastic change.

The penalty has made a major impact on eBay’s bottomline, which is probably what led the ecommerce giant to reconsider its partnership with Google.

“Within a month of the penalty reports, RKG found a large volume of eBay search ad clicks starting to take place from ads served through the Bing Ads platform instead of Google AdWords, through which they had been served for years,” RKG reported.

Apparently, the traffic that was transferred to Bing is exclusive on mobile search, as desktop ads are still served through Google. Historically, eBay has run syndicated search ads at the bottom of its search results pages to monetize its site traffic and capitalize on its content.

Search Engine Land suggests eBay may still be testing Bing as its mobile search ad provider. However, if the ecommerce firm permanently dumps Google on mobile search, as well as on desktop, that would definitely take away a huge chunk of Google’s ad revenue.

But, that’s not the sole threat Google is facing on the advertising front. Amazon, another ecommerce titan, is allegedly working on its own native advertising platform and is planning to remove Google ads from its pages.

Meanwhile, Bing has been winning partnerships with Apple, Facebook, Twitter and Amazon, effectively boosting its search volume. And with this new success with eBay, mobile accounted for almost 40% of Bing’s ad clicks in Q3 (up by 11% from Q1).

With all these circumstances surrounding Google, should the search giant start to worry?