Once when I was asked by a friend to do their shopping site, I suggested ready made paid solutions like Godaddy.com, Amazon or even sell it through ebay. There are packages available from Godaddy and from few other websites that are paid and that give you the complete shopping cart funtionality.

But I always liked free.

Recently, I came to know from my friend about the package osCommerce. I knew about this, but had never really appreciated it. This friend mentioned that he purchased a template that is integrated with osCommerce for $160.

I started searching for osCommerce templates on google. I found many free ones. One of the template that really attracted me is called the GG9 - Free lightbox osCommerce template. It is available from the below link:
http://www.badeziner.com/2008/07/12/gg9-free-lightbox-oscommerce-template/

Actually, if you want to build a shopping site, I would recommend the following solutions:

Fully paid and payment integrated:
- Godaddy (shopping cart is available from $9 to $42 at the time of this post)
- Amazon (Search for webstore from amazon and you will find). Here is the link:
http://webstore.amazon.com/

Free solution:
- Find a good osCommerce template for free. This usually comes with the osCommerce package that is also free.

If you just want to sell stuff and dont really need a website or personalized solution:
- Amazon.com: Open an account and start selling
- Ebay.com: This is a bidding site. But you can sell products at a standard price.

Here is the link to osCommerce:
http://www.oscommerce.com/