Can I use marketplace free assets in a completed project?

Is usage of marketplace asset legalized in completed project ?

Normally, yes as long as you use them in an Unreal Engine project. But please refer to the particular usage rights of the assets you want to use.

At least for the free assets from Epic I’m pretty sure that they are free to use in commercial projects. Here is also a post that says the same thing: Question on Epic asset usage in Marketplace content - Marketplace - Unreal Engine Forums