The private stores plugin forces your shopping cart customers to have a account (via the Member Logins) feature in order to purchase products.
It also allows you to specify budget amounts for each secure user. You can configure these budgets to be per-purchase (so it acts sort of like a "max cart limit") or concurrent (i.e., they can come back later and buy more stuff until their budget hits zero).
Budget functionality is highly configurable (e.g., you can set budgets to reset every month, every purchase, every year, etc) and completely optional (you don't have to use it if you don't want to...you can just use the member login requirement functionality).
If you want to only allow people registered with you to purchase products (as in the case of a wholesaler or private accounts) then this plugin is for you.
How it works:
1. You install it.
2. Your shopping cart customer adds some products to his cart
3. He goes to check out
4. He's prompted with a login screen
Version v4.9+ required
Make sure that you are running v4.9 or later.
This add-on will not function properly on any version earilier than v4.9.
How to find out what version you're running
Log-in to manage your website with Pro Edition (i.e. http://example.com/sohoadmin)
and look in the bottom right corner of the admin tool window (ie. in the footer).
How to update your version
If you're running v4.8.x or earlier than you've got to first update to v4.9 via Fantastico.
Once you're on v4.9 you can update to the latest build using v4.9's built-in Software Updates feature (Main Menu > Webmaster > Software Updates).