This is why copyright is so messed up. If I take an photograph of a building designed by someone else, can I sell my photo freely?
Second there is a major distinction between commercial use and fine art. Selling an image on a stock photo site to be used in an advertisement requires a release from any models, or from the sculptures copyright owner in this case. If they were selling a limited number of fine art prints they would not need a release.
But most likely it's not, so your image would almost meet the Fair Use standard. It's derivative.