hmmm... I don't fully understand your post are you saying that anything purchased from you is not available for commercial work? Any "clauses" should be made upfront, prior to purchse. If that is the case that should be placed on your items pages, I know that I will ~never~ purchase an item that is not available for commercial work. There are plenty of high quality freebies for my personal galleries. Or, are you talking about redistribution? No one has the right to redistribute even if they purchased it, without consent, that's as much a warez item as buying Poser and giving it away.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo