Can we use stuff from pre-samples like rocks and trees, tables, glass etc for use in our own projects?
For commerical use??****
AFAIK, no.
However, once they open the marketplace up you can purchase other people’s assets and use them in your projects for commercial use!
You can do whatever you want with it as long as you give Epic 5% of your revenue.
From the FAQ:
What can I do with this free content?
You can use template games to build and ship your own projects. We encourage you explore, deconstruct and modify the sample games and example content as you like. You can use this content for your own game projects, too!
Any content you have received related to Unreal Engine 4 Per-Seat is free to use in your game.
So, if you have access to other content through your employer or direct access to Epic’s Perforce for other reasons, content you get there you CANNOT use.
However if you have purchased a seat on UnrealEngine.com, and that download (and related downloads) are your only source of UE4 content, you are free to use all of it.