eBay in its announcement last June 26 has explained that all listings, regardless of how they were listed through mobile or computer, will automatically be mobile friendly.
Before sellers need to use the Mobile Friendly Checker tool to make their listings mobile friendly this helped them add the viewport meta tag, which according to user ShipScript of this forum: “The viewport meta tag does not make a page mobile friendly. The viewport meta tag is only an advisory to the browser, telling the browser the page has already been made flexible, so don’t try to shrink the page.”
eBay then stated that since the update, “the viewport meta tag will be added by eBay at time of rendering the View Item page on mobile devices.”
eBay has also clarified that existing and new listings will be affected by the change. Sellers no longer need to update their previous listings regardless if it had the viewport meta tag or not.
Lastly, the Mobile Friendly Checker still exists for listings with embedded images on their description. “The tool will warn sellers if the embedded images have defined a fixed width that is not mobile friendly.” For sellers who do not have embedded images on their descriptions, there would not be a need for them to use the tool.