Say you only sell to wholesalers, or only members of a certain group for certain items.  You can restrict access to who can view your Item Pages/make purchases by 1) Setting the correct Access Settings you want on a Page, as outlined on "How to Restrict Access to Members, or select Members/Groups", and then 2) Choosing that page on the E-Commerce > Add/Edit Category pages, choosing that page from the menu list for the option "Use Page Display Options from this Page", in the Individual Item Page Display Properties section.  Please note that the Page you create for the Individual Item Display Properties on the Pages > Add/Edit Page may be set to Inactive there to only use for this purpose.

If you want to password protect/restrict access to your Item Pages and Item Category Pages, just make sure you set the correct Page access on the Pages > Add/Edit Page, and copy/paste the Placeholder Text for the Item Category there (refer to Configuring & Adding Items/Item Categories).