When you deploy a project like a game, the asset files and their source are not exposed, but when you sell a Scene, you are selling exposed assets and their source. You really cannot sell a scene with someone elses content, unless they are free and this information is explicited somewhere to be confirmed by the MP crew. You can sell assets that are plugin dependent and this information must be included in the package information and the project file. If someone buy your scene and put an extra house to make it more appealing and they are able to sell it, imagine that this person will receive all the profit for everyone else’s work… this is the basicf thought behind it.
Long: You sell it with rights to assets, i buy, i don’t have rights to use those assets. I now am liable, if i sell my game, i can now have a lawsuit by the person i didnt buy the asset from.
Example: you made the rocks in your item. I buy, getting trees foliage, rocks you made, materials, etc, all paid from the marketplace.
I use all that in my game. Everyone (except you) can make a lawsuit against me for use of each asset. I then find where you live and send you a strongly worded letter. You dont want letters do you?