Multisite Help: Invidual stores with MarketPress

Question. For the end goal to be each user has a unique store, does each user have to create their own site or can a site be created under the admin user?

I noticed that as an admin, I can create as many sites as I want under my account. Can I use different shopping carts in each of these sites?

I guess I'm confused on how a site gets built within WP MultiSite.

My end goal in the user experience:

Click Register
Register form comes up with username, email, custom field questions on the front end
After registration the user sees their manage account page on the front end.

In the backend, this needs to happen:
-A unique store is built
- a unique page is built that shows this store and their profile information
- a manage your account page is built on the front end where users can add products
- a list of transactions from that store is built on the front end.

I have created my own child-theme based on FrameMarket and GridMarket to eventually do all of these things. Right now I've just modified the feel and look and already created a front-end CREATE MY PRODUCT form.

Any help on the registration and backend automation during registration would be appreciated!

Thanks,
Giovanni