15 Best Free Open Source Ecommerce Platforms | Web Resources | WebAppers
various google searching will turn up lots of reviews, best-of lists, etc.
I'd recommend start by building a linux machine in virtual box, setting up the LAMP stack, and trying a few of them to see what meets your needs. Focus on the tasks that you will have to do lots of - it may be a pain to set up an initial template, but you only do that once in a while. Managing your items, associated photos, processing orders, processing payments, etc. go on daily. Narrow it down to 2 or 3, then see what kind of user and developer communities each has. Then pick one, get a linode.com server, and set it all up for real.
In reality, I'd fire up a text editor and start coding.
But that is just the back end - what about what customers see?
For the actual web design - colors, fonts, etc. - I'd get a (semi)pro to set you up a template/css file/etc. Check the graphic design program at a local community college, etc.