Hi!
#1. you can render previews in any software that you want. It is preferable to mention render program in description, especially if the model was made not in the same software.
#2. absolutely NOT! You can't resell any models that aren't belong to you, no matter if that model is free or if you paid for it. There are only few exceptions - you can resell model if you aquired exclusive rights to the model or if the model was obtained under public domain licence. Such models are very rare, so it's pretty safe to say that you can't resell any model that you've downloaded from cgtrader or from anywhere else.
#3. same as #2 - you can't resell models obtained under royalty free licence, no matter how much or how little you have modified it. By the law, you can't put on sale your model if it contains even single polygon from models mentioned above. Royalty free doesn't mean that the model is free to use whatever you may please - it simply means that you don't have to pay royalties to the author any single time you use the model (in allowed manner).
All those statements are exactly valid for textures and other digital assets as well.
Have a nice festives season!