If you have actually seen these things you should report them to the admin as it is indeed illegal to take someone else's mesh, alter it and then sell it or give it away under your name (unless you have express permission). Most modellers use the base figure as a guide to making clothing. For example, I use Rhino to make my clothing and I can create contours on a model to get the basic shape for my clothes. I do NOT at any time use the actual mesh of the base model itself. If you are going to make clothes for Poser, I suggest finding other tutorials (there are many here and at Poser Pros) to explain what I mention above, as it is a perfectly legitimate way to do it.