Being legal does not mean that something is ethically or morally right.
The artistic community (as do all communities of any sort) has it's own set of ethics that extends beyond what you are legally allowed to do. If you cross that line, you may not face prosecution, but you can expect to be persecuted relentlessly, night and day and all the hours between.

As far as selling materials, It depends. I could hit the random button then turn up the reflection and metallicity 300 times, then package and sell them. For that matter, I could get a free tree generating program, load in the sample trees that were included for free with the free program, up the leaf count a tad, export the obj's and sell them. All perfectly legal, but I'd personally feel like the lowest piece of crap that ever turned on a computer, I mean really, how much integrity and self respect would you have to flush down the toilet to do something so basely immoral?
But then if AgentSmith were to release a material package, or Zhann, she has several material packages (which I highly recomend and are available at http://www.zhannsartwerx.com/ ) Then we'd know that they'd spent countless hours of effort and have applied their years of experience and skills to the creation of the content and we'd feel that they were entirely justified in charging for their time and effort.