I'm currently investigating a number of options for setting up a basic e-commerce site. My first inclination was to try and push the limits of WordPress. There are a number of shopping cart plugins that I could use to make it happen, however, recently I've discovered another open source e-commerce solution, Magento.
The advantage that Magento appears to have over the WordPress option is that it has been designed specifically with e-commerce in mind. This is a good thing, however, it also means that setting it up, and configuring all the options correctly can take a lot of time.
Magento lets you set up various options for products, ordering options and tax rules.
If you're interested in Magento, you can download it here:
I found this cool tutorial on setting up Magento from scratch which is a must read for anyone considering using it as a shop front CMS.
At the moment I'm leaning toward Magento for my project. My only reservation being, I'm not sure I can find the motivation to pick up yet another CMS...