Questions regarding copyrights of textures.

In general images need to be altered so that it does not display as the original, It needs to be completely or partially unrecognizable.

Putting an image in a 3D frame and render it still will display the original image 100%, same case with book cover.

The original creator could for example claim you are using his design and public awareness of it to draw attention to your product without him having granted authorization to do so.

Sharing it for free would be different story if that image would not directly or indirectly be used to draw attention to some commercial activity's (difficult to prove because CGtrader is a commercial platform).

Textures is a different story as it usually incorporates into a totally different output image.

Besides downloading, there are texture generating software too. Filter Forge for example (crashes a lot on my mac though).

Every texture stock site has terms of use. You should read it carefully before using textures. You can sell your models if you used textures from textures.com: http://www.textures.com/terms-of-use.html

As for random textures from google, usually you can't use them commercially, unless it is stated otherwise by an author of texture.