If you are using a desktop browser, you should be able to hover your cursor over the "Hi [firstname]!" link in the upper right corner of an eBay page to reveal a drop-down with a sign out option.
If you are using the Android version of the eBay app you should be able to tap on the three-bar menu icon in the upper left corner of the main eBay page, then tap your username to see an sign out option. The iOS app is slightly different, I believe.
If you clear your browser cookies or app data, that should sign you out if you are having difficulty otherwise. If you are using a public computer, starting an eBay session in a private or incognito browser window will allow you to close the window and end your session without having to remember to fiddle with cookies or signing out.
