Hi Ashish,
The basic idea of shopping cart is to let the user add items in it and at the end pay for all the selected items. Here in this case the user can keep on adding "order line items" to the "order" and finally he/she can pay for it once done.
So ideally the moment the user decides to prepare an order, there should be an shopping cart ready for him/her.
Now its up to the designer to decide where he wants to maintain this shopping cart. in
ejb (SFSB) or in the HTTPSession bean.
If you ask me, I would say go for HTTPSession.
thanks
Tanveer