1. Get something cheap or free to start with like Wordpress and an ecommerce plugin
2. Prove you can sell your stuff and break even or make profit on Google AdWords and Google Shopping (pointless selling stuff if you can't make a profit from it)
3. Once maxed out there test out Bing ads and product ads (USA only at present), and Amazon Sponsored products
4. Reinvest your profits into improving your cart where you can see any deficiencies on what you have, based on what you can afford

Although I don't (yet) discuss carts in it, my Amazon book "The AdWords Bible for eCommerce" may help.