The UE4 Launcher has a Marketplace currently, where we are releasing content for download and use. The plan is to allow users to submit content in the future!
It'd be cool if it was a built in feature but I'm sure content creators can just make a video of the asset, or in the case of Unity, just make a scene with the asset and release it as a webplayer demo. Personally, I'm more curious to see what this will mean for indie games. I can already imagine the plethora of AAA quality…