Tran Ngoc Tuan Anh

This plugin adds rel=”nofollow” to all external links in post content. So, your flow of PageRank can not run to other websites and your blog’s SEO is improved.
The internal links are unchanged.
Requires PHP5.

External Nofollow

Dan Griffiths

Adds a robust third-party product system to Easy Digital Downloads. When activated, adds new fields to the Download Prices meta box allowing you to specify redirect details.
External Products is based on the extension of the same name by WebDevStudios.

Easy Digital Downloads – External Products

Was looking for exactly this: an option to add third party plugins on my own EDD shop. 

Perfect

Adds a robust third-party product system to Easy Digital Downloads

External Nofollow vs Easy Digital Downloads – External Products