Online store suggestions

Okay, I have this store I'm doing a website for. They sell gaming cards like Magic: The Gathering and such. He asked that I build an online store. I have no intentions on recreating the wheel (and I hear it's quite a bad idea to attempt to) but I need some guidance on what open source software is out there that is easy to create a template for and such and possibly easy to use since the owner isn't the most computer literate person you'd come across. At least not in this field.

Suggestions would be appreciate. :)