I mean is it completely mine and sell the the game with the asset i bought on unreal marketlace?
if the question is: “i wan’t to sell my game, can i use marketplace content?” yes
in the FAQ:
“What can I do with content I obtain from the Marketplace and Learn tab?
Besides using this content for learning, experimenting, and prototyping, you can also ship it in your own products, too! However, you can’t sell or sublicense Marketplace content to other developers for use in their products, e.g. via website or e-commerce mechanism built into a 3D development tool.”
FAQ link
It isn’t yours, but you’re allowed to use it and sell your games with it in!
In basic terms, the creators of the content have traded you a license to use the content. They still retain copyright, but you’re given the right to utilize their content for your own needs, albeit with limitations as above mentioned by veccher.