If you have a product there is no reason why you shouldn't have an online storefront. Making your product available online will open your sales market to a global customer base and give you the ability to make a sale at anytime of day or night. Often seen as the "holy grail" of the online presence, an online shopping experience can be obtained using all open-source technologies as well.
Magento
The leading open-source online shopping cart technology today. The breadth of functionality available to you is comprehensive, even supporting multiple websites and stores from one installation. There are a great selection of community modules available as well.
OsCommerce
A long-running contender for best e-commerce package (had started development as early as March 2000) OsCommerce has many crucial features to run your store.
ZenCart
This project is an off-shoot project of OsCommerce with similar benefits, but features to help differentiate itself from its roots. In particular, ZenCart supports a proper template system.