For icons, most people use FontAwesome or similar icon font libraries. Demo images – usually PhotoDune or other stock photo marketplaces, with an appropriate license.
It could be that Themeforest is much more well known than it was in 2009. With far fewer members, it was much easier to find someone who was new to the marketplaces. Now, it is likely that many of the people you are sending are already members, or have otherwise visited the marketplaces before clicking your link.
And here’s the affiliate program rule to confirm:
If you have visited one of the Envato Marketplaces before and then click a referral link, it doesn’t count
Anyone know of a good plugin for 1.1 API that provides a shortcode, instead of a widget?