Yes, I'm writing one right now. Why don't I use an existing one? To begin with, there is a lot of variation in e-commerce apps, so an off-the-shelf system almost never does what you want. Customizing an existing one would be possible, except that most of them are very poorly written. For me to consider using an existing one, it would have to

• use strict and warnings
• have a test suite
• run well under mod_perl
• use a good templating system (and not one that was invented for the project!)

I could go on, but that already eliminates the ones I've seen.