Ed Foster notices something that has been at the fringe of my consciousness for a couple of months: you can't easily log out of some of the big e-commerce sites:

"Over the last few months it has become very difficult to sign out of a session from sites like Amazon and PayPal," the reader wrote. "The 'Sign Out' or equivalent link that for years was at the top of nearly every page is now missing from nearly all pages of those sites. Even the most obvious page where a sign out link should be -- the page acknowledging completion of an order -- offers no way to log out. Amazon and PayPal have turned things upside down and instead of closing a session, they now want us to remain logged in after leaving their site. Why would they do that? What good does it do Amazon and PayPal when their customers minimize the browser or surf to another site while signed in?"

This is somewhat of a concern on a machine at home, but it's a huge problem on any shared (work, library, etc) device. I think the safest answer is simply to not shop online using a device that may be used by other people - not that anyone will follow that advice...