Kimbo and G,

G you taking a process view (almost a DFD breakdown approach). I'm putting my money with Kimbo this time.

Usecases are NOT screen related, rather they are GOAL related. Most of us would have a SELECT/BROWSE Product and/or PURCHASE Product use cases.

If I were to buy products online, I'd normally BROWSE/select products -- and most of the time I dont PURCHASE. However, if I decide to buy a particular product (after some comparison or direct instructions from my wife) then I'd PURCHASE Product(s).

Use Cases are not business requirements; however Use Cases are BEHAVIOURS between actors and systems (computer or business) that are governed my the Requirements. 

Since usecases are disjointed, a user new to a site might not know the workflow to purchase an item. For this reason G and Kimbo many online purchase systems show/imply a workflow.  A good example is Flightcentres with their implied "Search.Compare.Book" workflow (see www.flightcentre.com.au ).

warm regards,

K