Evidently I have too many house files in my Library. I started a new Sims 3 folder, loaded a saved game, and was able to export the house. So now, it's just a matter of putting each house in a game, saving the game, and moving the save file to the new folder. Then, export it. Did I say "just?" It's a big job. That will be a lesson to me to export my houses right away and then delete them from the library if I'm not going to use them for a while.