DAZ3D explains license for products in this link, some products offer additional license options, others it's assumed. If you are going to publish the product in the public, then you should buy it, otherwise it pretty much is copyright infringement.
Read:
DAZ3D End User License Agreement
That should answer your questions, and limitations of what you can do with the products.
I'm sure renerosity and renderotica have a similar ELUA as well.