Zencart is excellent, still being developed, and very well supported
throught the Zencart Forums. It is also worth buying the book IMHO.

Take a look at Protx as a payment gateway and here for modules that
link into it so that you can accept credit cards for online payments

http://dev.ceon.net/web/zen-cart/protx_direct

Alex Threlfalls (Cyberprog)hosting services also complete the package.

All in all it works well for us.