partially related question about cgtrader etc

So ive been scrolling through a couple sites looking at models when i came across a kid, ( wont mention until asked)
who has been trying to sell others people work ( mainly tutorials on car paints and environment scenes) that he only did partial changes

would this considered legal or what would it be as i believe that they original creators should at least get partial credit or some royalties as he is selling them for a fair price
links to the content i found the was on the list: