You should talk her out of it. A half-hearted site will cost you more than you will make. There are 100’s of stores like this online, and you can be sure they had money and expertise to get there.
To have a chance at making it, you’ll need to hire someone to design the website, someone to setup the shopping cart/merchant accounts. You’ll need someone familiar with search engine optimization, and someone with experience running online marketing campaigns. You need traffic, and the only ways to get it cost money.
Try to talk her into using a website as a way to market her current business. A do-it-yourself site is fine for this. Just a simple presence on the web, the location of her business, what she carries, contact information, specials, etc. This is a much better use of resources, in my opinion.
Some ISPs, like GoDaddy, offer template-ready merchant sites for very little cost and very little setup. Whether it’s cost-effective would depend on the amount of sales and traffic, but I don’t think it should be dismissed so lightly, especially if you are halfway Internet- and computer-savvy.
OTOH, to compete with the big boys, you may have to spend multi-bucks for clicks, like with Amazon or Google. It seems to me that a specialty marketplace might make more sense than a general commerce one, but that’s only my opinion.