There is no e-commerce capability here. There is no secure payments system here at wordpress.COM and you cannot add a shopping cart to sites hosted by wordpress.COM. See: Moving to Self-Hosted WordPress
https://move.wordpress.com/
E-commerce plugin options have been removed here at WordPress.COM. Secure payment systems are not available here at wordpress.COM. You need a wordpress.ORG install. Please read this https://en.forums.wordpress.com/topic/e-commerce-22?replies=17#post-2575955
Provided you are selling only what you yourself make or a service that you personally provide on a wordpress.COM site you have two options.
(1) PayPal - Know that the only instructions that will work for posting a PayPal donation button on a WordPress.com blog are found here http://en.support.wordpress.com/paypal/
Note that the PayPal email link is not supported and PayPal customization options are not supported either https://en.forums.wordpress.com/topic/paypal-button-41?replies=12#post-2517229 This is not likely to be changed any time soon, if at all.
(2) If you set up an eCommerce site somewhere else online you can create a custom menu http://en.support.wordpress.com/menus/ and include a custom link to that site in it. http://en.support.wordpress.com/menus/#adding-custom-links
Some bloggers create a contact form https://en.support.wordpress.com/contact-form/ on a page and after the people have made a PayPal donation they provide the link to a private page for downloading files. Maybe that will work for you.
http://en.support.wordpress.com/pages/
http://en.support.wordpress.com/pages/page-visibility/