That's not allowed and will get your blog suspended.
There is only one advertising program called WordAds at WordPress.com and it’s for blogs on their own domains. This is the homepage for the WordAds program: http://www.wordads.co/ There you will find the application page as well as FAQs.
Aside from a blog being accepted into WordAds, no blogger initiated advertising, retailing or reselling the work or products created or services provided by anyone other than yourself is allowed on free hosted wordpress.com blogs. E-commerce transactions via shopping carts and the like cannot be conducted on free blogs from and being free hosted by wordpress.com
On free hosted WordPress.com blog we can use a contact form and/or a PayPal donation button for sale of only products or services created or provided by ourselves.
If you do require an ecommerce site, shopping cart, advertising and/or affiliate links you can hire a web host and get a free software install from http://wordpress.org Refer to > http://support.wordpress.com/com-vs-org/