To be on the safe side, assume that "non-commercial use only" means no renders using those products may be sold. You can always contact the creator to see what arrangements can be made. You are never allowed to redistribute the item itself, especially not commercially, without the explicit permission of the creator. Since this site has members from all around the world, not all of them native English speakers, it's difficult to be sure what they understand "non-commercial" to mean. I know that Kozaburo requires permission for his items to be used in commercial renders, thus the "non-commercial" for his items. I also know that some Free Stuff creators are simply trying to make sure that the items aren't sold but don't have a problem with them in a commercial render. Sometimes the readme will clarify it. As Kelderek pointed out, it's not just a matter of what is legally enforceable, it's also a matter of ettiquette. These people are spending their time and their money (bandwidth) to create things for us to use. It's only polite to use the items as they ask. (I'll see if there's something we can do to make the upload area more clear.) Cheers!