Good question! Other then a fully administrated portal (like the current marketplace) this could potentially also be done by users themselves. What i mean is not only to rate and comment an asset, but to also “confirm” it by the given information of the uploader. If the uploader provided source files (as mentioned - e.g. blender files), then it should be pretty much clear that this release is at least free to edit (even if the uploader picket it from another website).
But you are correct … To ensure proper content, it must be edited and eventually maintained by administrators. This could potentially also be done by users of the site with a wiki-like permission system.
But anyway … if you want to actually create a game and use these assets, you wont use anything provided by mister anonymous unless the included files proof themself by it’s source files 