Forgive me for not sharing sooner, but I've been frantically working on getting a Modx-based storefront of my own released. Just turned it loose on Monday, with probably a bug or two, but we were already late on the Christmas season. Take a look at
http://girl33.com/shop. Pleare help me and report any bugs and ideas, ok? If people are interested, I could now possibly set up a demo site and provide manager login info to some folks. I am not the most thorough or knowledgeable out there esp. when it comes to security and error checking, not good for e-commerce, so would be most happy to share with the brilliant Mod-xers around here.
Features: All TV-based (10 TV's so far I think).
Expanded registration table and signup form, with user account editing. User id is email address.
Categories based on page tree, with no theoretical limit to subcategories
Discount by product or category, based on quantity or total value ordered
Product options (uses a child page, with a simple boulean checkbox TV to mark it as an option)
Maintains stock levels
Most content, including product listing pages, is template-based using chunks.
Real-time U.S. Postal Service rate lookup
Paypal IPN based on class found on phpclasses.net. Not working perfectly, but getting the job done for now.
Limitations: English, U.S. currency, Paypal and U.S. Postal only at this time. Of course no reason more options can't be added--something I see Susan is already on top of.
Misc. notes:Adds new tables for orders, ordered products and paypal ipn (at this time they're in a separate database, but will probably just integrate them.
One minor core hack, a simple function to set a TV. Otherwise all snippets and modules (for backend reports). Still creating some of the backend tools.
Product pages are currently added via manager, not a frontend tool like News Publisher--counting on the upcoming manager rewrite.

I like Susan's ideas and will be taking a look at incorporating at least some of them.
Because of the combination of snippets, TV's, chunks and modules, if this were to be publically released it would need an installer, something I would probably never get around to, if for no other reason than I'd never think it was good enough with my sloppy coding.
Modx is fun.