I am a plugin developer. My plugins are not available through Envato (but through my website directly, which works well for me) but I have already bought themes and plugins here.
Now, I am trying to make my plugins as best as I can, and often it means working with other developers, their plugins and their themes especially. That’s no problem and usually a pleasure except in the case they have a contract with Envato. It seems like authors here are not able to share their products with other developers (or with anybody with a free license), which makes it impossible to test. Result? The products don’t have a very good compatibility with other plugins. Every time an user told me he needs help and that the theme he bought came from Envato, I know I will not be able do to anything.
I am also thinking of adding a note mentioning that I can’t provide support if the WordPress of my client includes any product sold from Envato. That would be a solution. A better one would be that Envato would let its developers/users the right to share the products they made with other developers they want to work with easily and for free. That would benefit everyone. Or at least provide a coupon system so that your developers can create a free coupon for someone to get their product.