shopping cart and login system

hi there , i am about to build a shopping cart which will interact with a paypal payment system , the cart will use sessions to store the items and basket information before checking out and posting to the paypal form , what i'd like to know is would the cart require a login system to track users and to prevent ppl from making dodgy orders , if not can a session easily be hijacked at all ? better still is there anyone out there who has intergrated their users with the paypal login, so say they login to your shopping cart to start making payments they are alreayd logged into paypal aswell this would be ideal :D