Is it common to use preview stock images and google images as model textures?

That is clearly not common practice and plane illegal, you basically have fallen victim to a scam and are entitled to refund in a case like this. Report this designer to CGtrader so they can investigate this case, he will probably get banned if this is true.

Sad story for the rest of us that work hard to bring genuine quality goods to the community.

Using images from book covers or other images found online is illegal, it equals sub-licensing so in this case the license you have purchased is not valid (for the models that contain this materials).

I agree with iterateCGI on everything, except for one thing - he said such practice is uncommon, but i think it's very common. Much more common than you would think. Many people don't even realise, that simply taking picture from internet is already might be illegal, let alone deriving and/or reselling it. Sadly such cases are difficult and often impossible to prove.

By the way, even if seller took pictures of book covers by himself, it still most likely that book model can't be sold and/or used with regular royalty licence, because real book and its cover are copyrighted. You can use such model only for educational purposes. If it's happen that you purchased the model with royalty free license, you are already eligible to refund, because basically you were scammed.

Thanks for the answers so far! I find it interesting to hear others viewpoints on this, especially those of you who do model things.
I understand not all modelers take images from google or buy images without even reading the license first, but it does seem to be a common practice by many who don't understand copyright or licensing.

I've been looking through this site to find some nice different types of home decorations to use in my projects, but keep coming across items with artworks which I'm unsure of who made and if whoever made it actually approved the use in 3d objects for sale. Like for example, framed paintings, books (often real world books with real world book covers), real world cosmetic brands with their logo and brand name on them etc.

I come across these so much that I sometimes think, maybe they got a sponsorship deal with a brand to make their products into 3d models or something, who knows.

If I will use these in my own commercial projects, I will want to be sure of that I myself aren't violating any copyrights, which becomes a problem if the texture (or brand/logo contained therein) is owned by someone else.

At the same time, I also have found models that, instead of simply copying the exact real world item and selling it, they make up a whole new "pretend" brand, which I think is nice to see. Like I recently found a set of soda cans with made up names and artwork for example.

Paintings, images and book covers are a definite no go but regarding most of the other real world items I think we somehow collectively agree most brands not see damages done to own real products or its sales thereof due to the existence of these digital copy's. They would probably make demands if a digital copy gets sold to masses in some digital world like a game but 3D stock seems to be different in that regard, but technically its copyright infringement.

Probably general consensus is, if the activity around the digital copy in some way supports the real world product and its popularization thereof, then its ok. If it undermines or influences the brand negatively or the sales of the digital copy is significantly large, then its obviously not ok.

This is not written somewhere but it seem most take this approach to it, probably safe to say its a gray area. Anyways, discussions around digital copyright are ongoing, for example EU has recently approved the “EU copyright directive”, it will have massive impact on how Youtube and social media is used, I would not be surprised new rules like this will eventually impact this market as well. If this trend continues then its not unthinkable in near future CGtrader probably needs to connect to some AI service that looks at all the contend uploaded and automatically filters copyrighted materials.