Google's argument has been dismissed by rival comparison shopping engines, which rely on attracting traffic from Google's search engine and are at the centre of the commission's case.
"As Google is well aware, neither Amazon nor eBay are rivals to Google's shopping comparison service," said Shivaun Raff, co-founder of Foundem, the UK comparison shopping service that was the first to file an antitrust complaint in Europe.
Instead, she said, the bigger ecommerce sites act as "merchant platforms" — full-service sites where third-party sellers can complete transactions. Comparison shopping sites such as Google Shopping and Foundem, by contrast, only take a referral fee for sending users on to full ecommerce sites.
Another sign they inhabit different markets is that comparison shopping engines list products for sites such as Amazon, said another person close to the commission's case — highlighting a symbiosis between the different categories of website. By contrast, comparison shopping sites do not list each others' products, this person added.
These can seem like fine distinctions, however, particularly as the boundaries between different online services erode. Even one backer of the case against Google admitted that trying to define the different players in online ecommerce was a "grey area", while adding that "they are not precisely the same market".
The second consideration that complicates the commission's case has been a fundamental change in how Google handles shopping searches in the years since the antitrust investigation began. Brussels had initially called on the company to stop giving preferential treatment to its own shopping results but in 2012 Google Shopping was converted into an advertising-based service. Merchants now bid in an auction to have their listings included in an index, then pay when users click on their products, making it similar to Google's Adwords.
"Circumstances have changed," said Gary Reback, a lawyer representing several US comparison shopping engines against Google. "We don't rank the same way we do on a relevance algorithm." As a result, he said, the commission's call for Google to apply its algorithm fairly risked confusing the picture and distracting from more relevant ways of limiting Google's power.
Some rivals believe another solution could involve showing another company's shopping results instead of Google's own advertising-based service if the former were a better deal for consumers. However, that would mean overriding a form of paid advertising on Google, going well beyond the commission's stipulation to be fairer in how its search algorithm works.
In an attempt to avoid thorny issues like this, former competition commissioner Joaquín Almunia spent years on a fruitless attempt to reach a voluntary accord with Google. The formal complaint his successor has filed will trigger a new and more high-risk stage in the long-running case.