I wanna know if I can use some assets from showcase project that we can download from the marketplace and use them in my project for a later commercial use? example if i want to use trees from the showcase “Landscape Mountains”. can i use them?
Well, you may only use it for Unreal Engine based products.
From the Marketplace Business Terms FAQ(https://www.unrealengine.com/marketplace/marketplace-business-terms-faq)
"What can users do with my content once they purchase and download it?
All content sold on the Marketplace is licensed to the buyer (who may be either an individual or company) for the lifetime right to use the content in developing an unlimited number of products and in shipping those products. The buyer is also licensed to make the content available to employees and contractors for the sole purpose of contributing to products controlled by the buyer.
Each such product built using content purchased on the Marketplace must be a standalone creative work (such a game, simulation, or video) in which content is merely a component and not the primary focus. For example, it may not be a content pack for distribution to engine users.
Can users modify, resell or transfer content once they purchase it? ?
Users are free to modify any content they purchase from the Marketplace.
All items sold are non-transferable, and users cannot sub-license the content for further redistribution, whether or not they have modified it. Users cannot buy your content and then distribute it to other developers to use in other projects, for example by listing it on the Marketplace or another store themselves."
Other links to consider:
has most of the details posted, but I wanted to clarify the point:
Any assets that Epic provides for free with UE4 may be used with your UE4 product. Payment for their use is covered by the royalty. Any product that includes those assets is also subject to that stipulation, even if it is created outside of UE4.
Let me know if you have any questions.