This appears to be a problem with our sso (single sign on) script. A fix is being prepared meanwhile there is a workaround in place and it should be possible to access the shopping cart once more.
Help with Ordering Checkout Needed
I am trying to order books from the book sale. I have 3 in my cart, but each time I click on either the Proceed to Checkout or PayPal Check out buttons, it takes me back to my own home page. I have tried Safari, Chrome and Firefox with the same result each time. My popup window blocker is off.