And even then, before the show is filmed, a contract is already signed and in hand on their house of choice, from what I understand. It’s a prerequisite for being on the show.
True. I've also heard you can predict the house they will pick on House Hunters almost all of the time because it will be the unfurnished one. The other two are picked by producers & buyers get to play act like they are interested.
(This applies to the U.S. House Hunters & I'm thinking might be different in the International version since those are often sold furnished).