I agree with Xena- selling "inspired by" products has always been allowed in the marketplace sites, provided the product does not use any copyrighted names or logos. I don't think it's "unethical" to allow these products... though I'm not sure why someone would take the time to make their own version of the APC when the one at Vanishing Point is "Aliens" branded and is free. :)
 
And the admins at PoserPros shouldn't put themselves in a position where they're telling merchants not to sell an item because a similar version already exists and is offered for free.
 
--John

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