Using free 3d prop models in environment models

As mentioned in the help:
"Can I sell a model I found online for free or use it as a part of my model? No. Even if a model is available for free, it is still protected by the copyright law. Quite often models found online for free are actually stolen and reselling such model or using it in your own model will result in copyright infringement and breach of our Terms and Conditions. If such model will be found or reported to us, we will suspend your model. In case of multiple or repeated infringements your account will be suspended and you will not be able to use our service in the future."

https://help.cgtrader.com/hc/en-us/articles/360015142637-Can-I-sell-a-model-I-found-online-for-free-or-use-it-as-a-part-of-my-model-

You could always ask permission from the artist and tell them your intention is to sell their model, however I would advise you make all the models yourself to avoid any potential copyright issues or having your account banned.