Honestly right now, there are a few options and ideas and none are set in stone. As a base, I’d like it to not be possible for freelancers to spend money and make none. So that lends itself to a tiered percentage perhaps, but there are other options. What I wouldn’t want is a system of membership or pay for ‘credits’ to post (sell/pitch) and then have no-one buy those items.
The whole point is to try and make it easier for freelancers to be in control, so up-front costs don’t help that really and certainly not if those items never make the seller any money after posting them.