Is it okay to sell my 3d models that are included in a 3d scene that Im selling

Of course, just state in each model's description that it is used in other of your models or scenes, and vice versa, that a scene or model contains models that can be bought separately and list the related models.

You can state on your scene that models are sold separately on demand.

Yes its fine. Many artist do that as well and even same models but in different sets.