Hi there!
WordPress.com offers several options for those who want to generate some revenue from their blog or site, including affiliate links, advertising, and PayPal and eCommerce integration.
You can read more about affiliate links and the types of advertising allowed on WordPress.com here:
http://en.support.wordpress.com/affiliate-links/
http://en.support.wordpress.com/advertising/
If you would like to sell your own work or solicit donations using Paypal on your WordPress.com site, this link will give you instructions for setting up a PayPal button:
http://en.support.wordpress.com/paypal/
WordPress.com Business users can integrate eCommerce features in their site by using one of our eCommerce partners. If you have a store at Ecwid, Shopify, or Shoplocket, you can showcase and sell your products on your WordPress.com site:
http://en.support.wordpress.com/ecommerce/
If you require eCommerce/shopping cart functionality beyond this, you might be interested in a self-hosted (WordPress.org) installation. There are no restrictions on eCommerce 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 between WordPress.com and WordPress.org:
http://support.wordpress.com/com-vs-org/
We have hosting recommendations at the following link if you are interested:
http://get.wp.com/hosting/
Cheers!