SE doesn't want you to sell it.
That's why SE makes it difficult to sell it.

People were using houses as a commodity back in the day, and SE said to stop it.
It's not like it stops people from selling their FC that has a house, but that's a sketchy grey area workaround that is probably not something you want to advertise yourself as doing.

Also a single character cannot buy up several plots anymore, SE has locked that option as well. They didn't take houses away from players that did, because they didn't do anything wrong in getting the houses. But these days you cannot.