Is there a link with your username? Like this: "Hi, C!" (sign in)
If so, you may be able to hover your cursor over the "Hi C!" portion to reveal a dropdown with a sign out link.
If not, try refreshing the page, or refreshing while holding down the shift key.
If that does not work try this link to sign out:
https://signin.ebay.com/ws/eBayISAPI.dll?SignIn&lgout=1
If you do not have that link handy, you can also clear your browser's eBay cookies -- that will effectively sign you out.
If you are using a public machine used by others, use a private or incognito browser window; then closing the browser window will end all your sessions.
