A shopping cart needs to be user friendly, to both the administrator and the online buyer. There are numerous options avaiable, each one more confusing than the last. The carts I have worked with are:
osCommerce, which is a free open source shopping cart. The disadvantage to this shopping cart is that you need to have an understanding of php and coding in order to make it look the way you want, and to do what you want. The advantage is that it’s widely used, has the most free addons and contributions, and gives you a lot of freedom of expression.
Another cart is LoadedCommerce, which was previously CRE loaded. This option isn’t free, is based on osCommerce with lots more bells and whistles, and still needs someone who knows what they’re doing when it comes to coding.
There are several addon options for WordPress, but I haven’t found an option that convinces me it’s a safe viable alternative.