It almost certainly would be a copyright infringement on almost all models, DAZ or otherwise to give an exported .obj model to someone else, unless it is your own creation. Almost all vendors don't put any restrictions on a render of the object if you bought the object, but you need to check the license agreement to be sure. I am sure there is something with a restriction, but I haven't run into any yet. Things in the Marketplace, I believe, give you full rights to commercial renders without having to give credit. A lot of the Freestuff requires credit to be given. Again, you should always check the license agreement.