A good discussion there!
My own opinion is that I would never sell something I painted in a workshop, nor would I sell the sketches I do from catalog photos. Those are lessons for my own painting education, at the lowest cost I can manage. I don't own the photos, so to me that would be totally uncool.
But what is it that they used to say "your own mileage may vary", taken to mean that peoples' opinions do vary so much on this.