Otherwise you can sell only products you personally create or services that you personally provide on a wordpress.com blog.
If you do not have that upgrade you can post a PayPal button http://en.support.wordpress.com/paypal/
If you have a shopping cart set up for your own products or services elsewhere online you can create a custom menu and include a custom link to it in the menu.
There are no restrictions on advertising and selling products you don't personally make if you hire a web host http://wordpress.org/hosting and setup 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:
If you don't have a username account at WordPress.ORG click http://wordpress.org/support/ and register one on the top right hand corner of the page that opens, so you can post to the support forums there and receive advice from WordPress.ORG bloggers.