Well, for one thing if you get it direct from McAfee you know you're getting the real thing. I checked the site and, yes, it says it's McAfee software and some of it is sold direct from Amazon (some comes from third party suppliers).
But after my recent experience of receiving counterfeit Microsoft software sourced through Amazon (Amazon were not to blame) I'm wary of special low-price offers like this. It may be okay, and for $11 it's probably worth buying. But there's always a chance that it may not be.
Will McAfee price match? I doubt it.
Edit : Plus - used software??? I think you'll find that most software suppliers don't like, I mean really don't like, multiple PC installations of one purchased software package. It's usually in the EULA. Don't go for one of those "used" copies, whatever you do.