In some cases, when you need to access account-specific information such as Purchase History or account settings, you may be required to sign in again for security reasons.
If you have not signed in lately, your sign-in credentials may be "stale". Sometimes explicitly signing out, possibly restarting your browser (or computer) and then signing back in might help.
Repeated sign-in requests might be a sign that your browser is set to erase cookies, or is preventing cookies from being saved, or possibly has some other cookie-related problem. In some cases, manually deleting your ebay.com cookies and then signing back in might help if the problem is related to a corrupt or blocked cookie.
Under your Account: Site Preferences there is a check box to keep you signed in (and another for the Community discussion boards) -- you may want to check that box and hit "apply" if you are having difficulty staying signed in.
