Zen Cart is a fully-featured shopping cart with an active support community. It's based on the old osCommerce engine which you can use to sell items over the Internet. Zen Cart can also be customized with templates to modify the way the shopping cart looks and various modules can be installed to customize the functionality of the shopping cart. Zen Cart is an increasingly-popular open source shopping cart system written in the PHP programming language and is available to DreamHost users as a One-Click Install.
Please note that you will still need to make arrangements for receiving and processing payments in a secure manner. View the SSL certificates overview article for further information.
You can visit Zen Cart's website for more details and to see examples of it being used.