It depends what build phase they are in. If they are considered a saleable unit they go to auction. If they are early pre-production they are crushed. Just depends on what phase of the prebuild phase they come out of. Early production or tool trial vehicles are all scrapped. They do get to a point where they can be sold.
Everyone should visit an auto plant at some point in their life. Come visit us in Michigan and go to the Henry Ford Museum and take the Ford Factory tour. You can see F150 production live. It will give you an appreciation for how complex vehicle assembly is. Having worked in the auto industry for 20+ years and being exposed to it you get a little desensitized