Re-using (free) 3D models from another seller as part of your bigger 3D model

I'm not a lawyer, but if you simply include a condition somewhere with your free download where you make it clear to the user how you intend for the free model to be used, then you would have a strong case against someone you find misusing your freely available item. Now I don't know if it is illegal for them to use an item you offer for free without this, but it is absolutely and certainly unethical either way. Obviously no one with good intent and ethical standards would sell an item containing someone else's work without at least asking for permission from the copyright holder first. That's just my take on it. BTW, this has happened to me in the past. I used to offer a lot of models for free when I first started out, but after a while, I decided I had to start charging for them because they were being used for commercial purposes I never intended, without permission far too often.

If you find someone selling your model as part of his scene without permission, you just have to contact CGT support and they will take care of the illegal seller.
It doesn't matter if you had offered your model for free or for a price.
Nobody has the right to sell or redistribute (offer for free) your model without your permission.

All sellers doing that kind of thing (putting models that they had "found" somewhere on the internet for free in their scenes) are risking being banned from the site.

I had similar case. Once I found my free model as element of paid complex item. I was no harmed. Can say more: that complex model looked good and quality... And I just leaved comment below that complex model (something like...): "Glad to see my free model as the part of your project"... And got the reply to my comment: "It's not your element! I made all parts with MY hands from start to end!!!"... And this reply triggered very very big disgusting. After it I decided to assign price to my ex-free model and reported this case to administration. Administration (all hails and thank them) suspended that complex model. And the same model was deleted from 3dexport after my request... I learned lesson and after all I don't giving models for free anymore (except when I'm giving something as bonus to my customers and buyers).
...and it's one of reasons why I never using free downloaded models as parts of my projects (but sometimes i'm using free models from internet as reference for remodeling or modifying).