e-commerce isn't possible here at wordpress.COM. For one thing, the code and programming needed to do it is not allowed here for security reasons and secondly, it is against the Terms of service. You can sell things you create yourself such as jewelry, crafts, art, etc. but you cannot sell things you buy.
For e-commerce, you will have to self-host a blog on a third party hosting service such as godaddy, dream host, etc., using the software from wordpress.ORG. Figure an average monthly cost of about $25 to $50 including payment processing and SSL certificate. There might also be some one-time set up fees with the payment processing people, and prices for SSL certificates can vary widely from place to place.