It definitely needs a 'bridge" to a more heavy-duty system, although I suppose a complex catalog could be created using TVs and filters and tags with Ditto. For more serious user management, the registration snippet could be upgraded with an "edit profile" addition.
But really, if a plug-in and module to connect to CPCommerce or ZenCart or even <shudder> OSCommerce were available, it would save a lot of complex work; ecommerce is not a simple feature.
Again, I'll emphasize that even with a more robust checkout and payment module, ShopX will never be suitable for more than a very simple small eshop, at least not without a lot of hacking, in which case it really wouldn't be ShopX any more
Don't sell yourself short. ShopX is a very good start towards a superior eCommerce system. It takes advantage of MODx's organizational, templating, and database structure to create the catalog and checkout pages, which is most of what an eCommerce program does.
I just completed a CubeCart site, and I'm about to do two more eCommerce sites. I'm going to try to make ShopX work instead of using a "real" eCommerce program. I've used several of those in the past (including ZenCart and <shudder> osCommerce), and frankly they all seem to be necessary evils at best.
My opinion is that 99% of the code of most eCommerce programs is templating and language files and allowing them to work with various gateways/currencies/regions/etc. If you don't need all that bloat it just makes your life more difficult. ShopX with different snippets for gateway options seems to me to be a perfect solution (small is beautiful, after all).