Yes I agree that osCommerce is really awesome and FREE. If you are a beginner programmer and don't know a very much about PHP you can still use this website very easily. However if you want to change the look and feel of this website you will need to change the source code allot.
There is another really awesome one that you can very easliy change the look and feel of the whole website and it is really easy to do so for beginner programers. It is called Digishop, but you have to pay like $300 or something like that, but in my opinion it is worth it. All you have to do is simple create the website you want in HTML and add a few special PHP tags in certain locations (many are given to you already) and then post your HTML file to a little applete that digishop created and it will automatically make the site look just like you want.
You can take a look at a test website I am working on now.
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Both OsCommerce and Digishop have really elaborate administration sides that help you list, sell, and create reports for your website.