Unfortunately, Envato has a rather poor affiliate program. You only make money when A) a new user signs up to their service and B) deposits money into their account.
Since most people using a tool for ThemeForest probably already have an account I don't expect to make much money. But that's ok, this was mostly built for fun and as a portfolio piece.
Yeah, this is an ongoing issue on the Envato forums. Most new users coming to the site probably don't make a large deposit to start. Like me, they started out with a deposit large enough to purchase a single file, or around $35. Later, when they decide they like it, they'll deposit more, but that doesn't reward the affiliate. I think they should have a smaller percentage, but spread out over the first X months, or X deposits.
Keeping on top of referral fraud is a bit tricky too, and gets worse the more complex you make the referral system.
edit I also remember that way back then when the code was redone there was no themeforest, just flashden and audiojungle. We were heavily focused on finding new customers more than directing people to specific files. So that might give you a bit of context.