WordPress.com is already running advertising on your blog. There is only one advertising program called WordAds at WordPress.com and it’s for blogs on their own domains. Aside from a blog being accepted into WordAds http://www.wordads.co/ 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. Paid content and/or sponsored content is not allowed.
The only very narrow exception is found here in italics:
Blogs that violate our advertising policy or fall into one of the following categories are not allowed on WordPress.com:
Affiliate marketing blogs: Blogs with the primary purpose of driving traffic to affiliate programs and get-rich-quick schemes (“Make six figures from home!!”, “20 easy steps to top profits!!”, etc). This includes multi-level marketing (MLM) blogs and pyramid schemes. To be clear, people writing their own original book, movie or game reviews and linking them to Amazon, or people linking to their own products on Etsy do NOT fall into this category. Here is a thread in the support forums that talks more about which affiliate links are OK or not OK.
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.
There is no upgrade you can purchase from WordPress.com that changes any of that. If you do require an ecommerce site, advertising, affiliate links, paid or sponsored content 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/