If this site ever gets built, it will be a ma-and-pa shop, so all of this would be a non-issue.
I don't know how Amazon or SitePoint handles this issue.
However, I can say with a high degree of certainty, that "Admin" accounts are intuitively different to anyone using them than a "User" account.
They usually lead to different areas, different screen, and have entirely different log in steps.
I would never allow an "Admin" to log in and shop from the main page of my website. Never!!
In fact, I would probably have a security rule stating that you cannot log in and shop from the main page of the website.
If you allow that, then what happens when "Joe Super Admin" logs in to his "Super Admin Account" at the public library so he can buy a DVD, and then he gets distracted by a pretty girl and walks off without logging out?!
Isn't all of this pretty obvious??
Yes, One Person can have multiple Account Types, but only One User Account can only have One Set of Security Rights.
You want "Admin" accounts to be segregated from "User" accounts on many levels...