I don't know why so many people go for those ecommerce packages that some hosts offer. You can purchase web hosting usually cheaper and with more space/bw and just install your own software. You could go with something like
zencart or
oscommerce since they offer shopping cart functionality out-of-box but I'd prefer something like
Drupal. I use drupal for my online shop. It takes a bit of getting use to but the more time you're willing to put into it, the better off it will look. It's all about what template you can cook up (or pay someone to do it). Drupal has an ecommerce module that I use and I think it's pretty good.
You can try out the default install of what I've named on
http://opensourcecms.com, they have them in the demo section on the left side. Drupal is in the section for portals.