Years back I was looking at an electrical job for a general contractor I worked with. The house was on the north shore of Long Island in a very exclusive community. The house was about 4,000 sq ft. Nothing special but it was right on LI Sound and you could see Manhattan on a clear day. Well I'm checking out the house and we are going through the basement. I ask him what's behind a door and he says he's not sure. I think you can guess where this is going, I find a 20' X 30' O gauge layout! Needless to say I took the project. While I was there I managed to get a train or two running that the new owner had brought to the house. He was not a big modeler but was intrigued buy this beautiful layout and wanted to learn how to operate it.
It was very well built, all conventional, block control. I figured that you could run 4-5 trains simultaneously. He was happy to have my help to get started. After I competed the project we spoke occasionally but I never went back to see the layout again.
So to answer the question- YES- I would buy a house with a layout already built. Selling the same would be difficult at best figuring that you need to be patient and find the right buyer.
Bob