But to answer your question, I shouldn't think so if they cannot add items to the card (because, these will be stored within the session) so aslong as the visitor to your site can only browse and doesn't have to do anything "usery" then sessions can be kept clear, for now.
You'll obviously need to use them when the user signs in and stuff :)
i guess not because you can always add the items once they logged in
but i don't know why you would not allow people to add the item before logging
then ask them if they have a account if they don't make one i think it would be very bad to not allow the user to add items some are just going to assume you don't have the item to sell