Error 502 is a server-related error and it’s unlikely to fix it on user’s end. Anyway, there are still some fixes you can try to make it work.
Issue can be temporary and reloading the page could be the quickest way to get rid of the error. When it does not work, take a look if it is not a global problem on Downdetector or other website like this.
When no one else is reporting the issue you can open eBay in different browser. There’s a high chance it will work. If it does you just need to make sure to clear cache and cookies in the browser where you encountered on 502 error.