I think if you charge people to upload content from day one, you won’t get enough content to drive demand organically. I think the website is going to have to be free to use for a period of time before you can start charging people. If someone doesn’t already have a vested interest in your team’s personal success (i.e. anyone who isn’t your friend and doesn’t follow this thread), they’ll look at it purely from a “how can I benefit from this website” point of view. If you have 100 users on your website, chances are my $5 sword model won’t make much money - not enough to truly care about making a login account and giving you my credit card information and paying 5 bucks. From the other side of the coin - if people aren’t uploading lots of assets (due to the website being brand new and you charging content creators), your buyers won’t sign up because there isn’t a specific item they “JUST HAVE TO HAVE” for their game idea.
I think charging people for anything up front (buying, selling, advertising, or whatever) will significantly hurt your membership numbers as you try to get off the ground, which at that time is more critical than making sure trash doesn’t get submitted for sale (because as an admin, you can spend 15 minutes each day deleting that content or doing whatever you want with it). You’re already taking 7% off each sale, so maybe just leave it at that. Promote the value of shared success and community so your community feels supported while it grows.
I also don’t think you’ll be able to roll out support for all of the game engines you want without help from a lot more than 20 people. It’s going to take hundreds (if not thousands) of users creating stuff to reach the financial success you’re attempting. A better idea might be to focus entirely on recruitment - do some searching on TurboSquid (and all the dozens of similar sites) and find their top contributors. Ask them to post their stuff for sale on your site and offer free advertising for the best content.
Think about the comparison - lets say you bring on a really good artist and he makes 15 high quality items for you, but I bring on a really good recruiter and he recruits 5 well known artists from places like TurboSquid. Now all of a sudden I have 500 assets to put on the store vs. that one artist who made 15.
I think your focus should be more on recruiting proven content sellers and ensuring your website’s backend and sales framework can handle your growth through 10, 100, 1000, and 10,000 users.