To ensure that the users are logged in first you should start out with Security Assist. It will generate the user login, and profile pages. If you would instead like to capture the network id you must first know what value this is, I imagine a server variable that would give you the ip address or something like that. In order for that to work you would need to have a table in your db that associated these values with the users they belonged to.
I think it may be easier to have a regular user login with Security Assist.
The rest of this is just having your product pages with the add to cart server behaviors on them so the users can add items to the cart. You would also want a cart display page so the user can see the contents of the cart. If you would like to add in a checkout you can do so by using the eCart checkout wizard on your cart display page.
This is just kind of a high level overview, please post back with any questions that you have about any part of this process.