Copyright infringement occurs when someone uses a copyrighted work without permission from the copyright owner. In the case of a sculpture based on a copyrighted painting, it is possible that the sculpture could be considered an infringement if it is substantially similar to the painting and the sculpture was created without permission from the copyright owner of the painting. The likelihood of the copyright owner pursuing legal action against the seller of the sculpture would depend on a variety of factors, including the significance of the infringement and the copyright owner's willingness to enforce their rights. The jurisdiction of the copyright owner would typically extend to any country that recognizes and enforces copyright law.
If you were to commission another artist to create a sculpture based on the painting, you could potentially be violating the copyright of the artist who has already created a sculpture based on the painting. This would depend on the specific details of the situation and the level of similarity between the two sculptures. If you are based in a country that recognizes and enforces copyright law, it is important to be aware of the potential risks of creating or selling a sculpture based on a copyrighted painting without permission from the copyright owner.