Screaming Penguin - Source Does Matter

I have used PHPShop minimally in the past. I was very alpha last time I used but has matured quite a bit. Its still not quite done, there are few needed "modules" yet to complete but it is a great project and great work has been done to date.

The current rev is 0.6.1 according to the site but 0.9 with the completed "core" is supposed to be out soon. Dont let all this 0.x stuff scare you though. Get it, check it out and then contribute to it. Also note that while there is work to be done PHPShop is already far along and in a very useable form (although I would double check security items and implementation details if you plan to use it in a serious production capacity, that is being done though.)

It is a much better alternative than paying somebody a ton of money for a product that is not really more mature (which is the state of most carts I have used) and it is an opportunity to pitch in. Dont start from ground zero, get PHPShop and help out.

Check out the tutorial series at devShed as well, pertinent stuff is linked.   http://www.phpshop.org/