While you can sell your work on your blog, no blogger initiated advertising, retailing, or reselling of work created or services provided by other people is allowed on WordPress.com.
This link will give you instructions for using a Paypal button on your blog:
http://en.support.wordpress.com/paypal/
If you require e-commerce/shopping cart functionality beyond this, you might be interested in a self-hosted (WordPress.org) installation. There are no restrictions on e-commerce if you set up a self-hosted WordPress.org blog instead of a WordPress.com blog, but there are some added responsibilities.
Please see this link to learn about the differences:
http://support.wordpress.com/com-vs-org/
We have hosting recommendations at the following link if you are interested:
http://get.wp.com/hosting/