Ask HN: Best e-commerce solution?

I couldn't find any threads relating to this topic, but if there already are, please point me in the right direction...

I was recently hired to redesign an online store for a local store. The solution they are using right now is terrible. I spend most of my time trying to figure out why my CSS, which works fine on my local server, doesn't work in their templates.

I was wondering if anyone had experience with pre-made ecommerce solutions, particularly these:

Magento (http://www.magentocommerce.com) Zen-Cart (http://www.zen-cart.com) osCommerce (http://www.oscommerce.com) X-Cart (www.x-cart.com)

Or would I be better off building a custom solution?