The whole point of selling various assets, including figures, props, textures and poses, is so that people can use them in their own works. Some are even sold explicitly as merchant's resources to be included in new assets for sale. A license may restrict the use of assets in certain contexts such as games as opposed to image or video production, but otherwise, if the license allows the use, then it is legal and ethical. I've seen commons licenses that explicitly require attribution, but I have never encountered a pose set requesting such.