Well, the way I understand it is there is no reason why he needs to credit the artist. Assuming it was a standard sale the artist has no claim to the piece any longer, and it's the owner's to freely sell. The only way it would be fraudulant is if the seller claimed it was painted by himself, or painted by someone other than the original painter. There is no need to explicitly state who painted it though.
Stating who painted it may increase it's desirability, but it does nothing to alter the quality of the item for sale. By not stating who it was painted by a buyer may determine a value for that mini that isn't caught up in speculation. In this case I would think it's preferable to advertise who the original painter was (to drum up price), but I don't see how there could be a legal requirement to do so. Doesn't a photographer who has sold an image to a stock photo supplier relinquish their rights to the image, and any sales thereafter?
EDIT: CMON may have a user agreement clause that prohibits posting work that was not painted by the poster for rating, but I'm fairly certain posting it for sale shouldn't be prohibited based on my admittedly limited understanding of business law.
