Yeah, the reason most people won't answer this question is because it's been done to death and usually attracts a scourge of "pick my favorite CMS" responses without backing them up with any facts. This site alone is certainly full of examples of that if you search "best CMS".
Back to your question...
You will find that pretty much all of the popular CMS packages support eCommerce in one way or another but as ralph.m said, we'll need some specifics in order to point you in the right direction.
How involved will you be with the design, development, administration and maintenance of the site? Where will it be hosted (shared server, VPS, managed server)? How much traffic. How many products? Who are the audience and is this big/small budget stuff?
Armed with some of that info, I think you might get some useful responses.