When it comes to eComerrce, its about money, and when money is involved your store should be secured. Its not only about your money, but your customer's too. I mean, their credit card info, personal and banking details, shipping and billing address, etc...
So, it should be a trusted software. I am not a seller myself, but I am helping many clients using this software. BigCommerce is the one I am referring to. Its not a free software, but its one of the best out there.
You can sign up for a free 15 day trial, and they will only bill you if you opt to continue. They have a wide list of features, and their source code is not public, that means hackers don't have access to their code, and hence can't find bug in their software.