What is the ruling on selling mannequins, the embodied forms on which clothes are displayed, whether they have heads or are headless?

It is not permissible to make statues, nor to buy or sell them. What is forbidden to manufacture, its price is also forbidden. This includes clothing display mannequins, because they are statues of living beings that must be disfigured. As for a headless statue, it is not considered a forbidden image, and there is no harm in acquiring or selling it if it is beneficial, provided that it is not used for something forbidden or if it is a three-dimensional representation of severe 'awrah (private parts) in a way that incites desires.