I'm sorry to say that they are right.

If any a model holds even just the smallest piece of anouther persons creation you can not refer to it as your own. If you are looking at adjusting a model severly and then trying to sell it under your own name you will get a lot of hate mail..

however.. If you upgrade the model with new add-ons or textures then you can sell only the parts/textures you have desighned for that model but not the model itself..

These days it is just a lot easier to start from scratch and build it from the floor up as I do.