You know what, just to clarify, I'm not sure I can state with absolute certainty what I said above about starting with someone else's work...

...but I'm pretty sure that, as long as I buy a copy of anything you want to start with, it should be perfectly fine for you to do work on it for me, if you see what I mean.
This way you aren't passing me a derivative work without the original artist getting paid, and it also means you aren;t selling a model (or derivative of) that isn't yours to sell.