How should I monetize a game development network app?

I am building an web app for game developers to exchange artwork, code, music, etc., however I cannot decide how to effectively monetize this network.

The most obvious way would be to advertise. I could place ads relative to game development (game engines and other such tools), but it does not seem like that would make nearly enough money.

The next thing I thought was to allow users to sell their various game assets, along with the normal free uploads, but I have limited time, and being able to enforce that seems nearly impossible. What would stop a user from buying a piece of artwork and then re-uploading it for free?

The third option I saw was to do something similar to Kongregate, where users can host their complete games on our site, we slap some ads on those, and then split profits with our developers, but again, would that be enough, especially when starting out?

Well first of all that's a great idea! Advertising is the bread and butter of the web. It allows free use of products with monetization. The use of let's say Adsense is easy. Just a snippet of JavaScript. But you can't really control the ads they put. You could weight loss ads on your site, no one wants that! On my startup (a social network) I wrote my own ad server. Showing only music ads (what we want!) on the site.

So I highly recommend ads. defiantly your own.
But don't spam users! Few ads but, good ads, you'll actually make more money that way.

Good luck!!

-Ben