Long answer: 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. Affiliate marketing blogs, paid content and/or sponsored content are not allowed.
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