Go to ebay.com in your browser. Scroll to the bottom of the page. If it says "Switch to mobile" in a black band, you are on the classic site version of the page and you can use the link to go back to mobile.
If it says "Mobile / Classic Site" then you are already on the mobile site version, and you can switch to classic using the underlined link.
Not all pages render correctly in one or the other format -- in that case the page may revert to the other format or you may be directed to another page. You can also try using the desktop version of the page by requesting that in the three dot menu in most browsers.
