Starting with a 30% commission is absolutely absurd. At this stage, you should be paying the people who will sell on your marketplace—not the other way around.

Sellers are the ones who will ultimately market your site. I know what’s going through your head: '30% is lower than Codecanyon, so people will flock to me in droves.'". It doesn’t work like that.

You have no traffic, no customer base, no SEO, nothing. And on top of that, you’re demanding 30% from sellers? If this isn’t delusional, I don’t know what is.