At least!

Hoewever, you need to add additional specifications. The examples you cited are primarily 3D sources. Sites that primarily sell 2d graphical content such as seamless textures and Photoshop/Gimp brushes would also fall under the "competing marketplace" rule, would it not?

Also need specifications on where the line is drawn in the case of off-site advertising of competitors. The majority of free 3d content sites out there that ARE NOT THEMSELVES marketplaces usually have link banners to marketplaces or have 3d commercial content sites links in their links page.

Also, jusrt my two cents, but I would prohibit links to file-sharing sites. This is because

1. Some file-sharing sites are plagued with illegal content. Even if the file in the link is legal, it will be super easy to find illegal content if it is widely available on the file-sharing site.
2. A couple years ago, ShareFile or ShareIt or whatever it's called was hijacked by Pirate Bay's blog for around 6 months (or was it 3?) meaning that Rendo freestuff downloads that led to a Sharefile now redirected to The Pirage Bay blog (which hosted a direct link to The Pirate Bay itself) ShareFile eventually took its site back, but all the old downloads were lost. This should NOT apply to file locker sites such as Skybox and Google File Locker.