I want to use unity’s 3rd package kit. I want to use that character model. Can ı sell my project at unity asset store if ı am using a model from unity’s own packages ? I will probably change textures of model so it will look unique.
You need to read the term and license of each 3rd party package otherwise you may face legal persecution. If you referring to the package that are in the Unity Registry, there is no restriction. Those package are not what you would call 3rd party but extension of the Unity Engine.
As far as I am aware, Unity does not provide any character (expect maybe the default one from the 3rd template). Also, anything you find on the Asset Store is not made by Unity. Unity package are on GitHub or in the Unity Registry section of the package manager.