If you are interested in a multi site/store vendor based system, we have developed the oscMall system. We are continuously updating our GPL virtual mall and have released the oscMall System for free to the general public. This Project is licensed under the GPL / GNU and is available to the public though our site. We are looking to improve the software through feed back and user contributions.
This system is based on the osCommerce system, but adds the additional feature for multiple vendors to be able to run their own shop under the mall system. We are always looking for new people to help in developments and testing with the mall system. If you are looking to add some additional experience to your portfolio, this is a good way to get started.