It depends. Supposedly you're talking about free models found on cgtrader, usually those are covered by the same general licence as commercial ones (you can found general licence conditions here: https://www.cgtrader.com/pages/terms-and-conditions). Basically it means that same rules applies - you can use models in your game as long as you ensure that you distribute those models enclosed in proprietary format and not accessible by end user, i.e. you cannot pack those models in OBJ or zip them in common archive format. However, be aware that not all free models are delivered under general licence, some of them has some sort of creative commons licence (CC) or even custom licence. In that case you must check what usage those licence permits.
And just to remaind, even if you purchaced model, that doesn't mean that you own it, you're just granted rights to use it accordingly to licence under which you bought the model.
