Am I understanding that a developer can now purchase the framework and create and sell themes (not child themes) of it?
This is not accurate.
I respectfully ask that people do not sell my work as their own, however, if you would like to purchase a stripped down version of the framework to apply your own design by Child theme, you can do that with Jump Start: http://wpjumpstart.com
Is it possible with some custom CSS to set the main menu aligned center?
I apologize, but this is a customization you’ll need to make to the theme. If you search the Internet, there are several different creative methods of centering items of an unordered list, although you find there is no real perfect solution out there. Everything is pretty hacky.
For CSS changes, it’s a good idea to use a tool like firebug or Google Chrome’s built-in developer tools to locate them and then put the edits either in your child theme or in the “Custom CSS” option on your theme options page.
Also, here’s a helpful tutorial on how to use Firebug – http://vimeo.com/20917974. The only thing I’d suggest not doing is actually making edits to the theme’s CSS files, as it shows at the end of that video. However, the video serves as a good introduction to Firebug and how it can be used to find what CSS is effecting certain areas of your site without ever digging around in any files.