I use Joomla with the Virtuemart addon to create robust webshops. I have written custom payment methods to support the Redirect online credit card transaction system of all (currently) supporting banks in Greece. These are (at the moment of writting) EurobanK EFG, Winbank, Marfin Egnatia, Emporiki Bank, and National Bank Of Greece.
In the year 2009 I implemented a "Webshop-in-a-box" service for SingularLogic S.A. based on the OpenSource CMS !Joomla. A couple of custom components and modules had to be implemented, along with an big extension of the VMConnector Web Service Library to allow connectivity with most SIngularLogic's applications (SEn, SEn4U, Business/Prime, Control, Orizontes and more...)
The Webshop has both B2C and B2B features, it comes with a full blown Synchronizer Application that takes care of uploading Items and Pricelists while trasnfering Customer Information and Online Invoices from the Website directly into the client's BackOffice Application.
With that package in hand a big list of WebShops have been implemented. A small part of the list can be seen bellow: