Typically, if you purchase a product it remains on sale unless the artist removes it.  What you download is a digital copy of the model.

However, should you require a unique model for your proprietary use, you might consider the '3D Modeler' forum in the job section of the forums...  Supply a description of your needs there and wait for a response.  Artists frequently subscribe to those job boards and are notified if there is a new post...

Good luck on your project!