I am always amazed by these creations. An unbelievable amount of time and work has gone into this, but it's just a show piece, and probably not worth anything to anyone other than the owner/creator. It's not practical or good for anything, other than maybe garden tractor pulling, in which case you need to invest in rear axles, tires, and probably transmission. Even if you bought it and decided to part it out, the engine is the only thing worth anything as the tractor has been cut up and modified so much.
I love to rebuild machines, and have been thinking of an articulating 4x4 cub cadet like JimT built, and I know it would be so modified it would fall into the category of not practical (as I've described above) but I know this going into a project like this and assume that it's for me only and not worth anything to anyone else. I wish him the best on trying to sell this piece of artwork, but I can't imagine anyone throwing $12K at it.