you are correct, I used the wrong wording.. no selling.. but the existing original building you put in storage is used to fuse the toadstool building. it's lost.. you don't have it anymore even if you built it from the construction site with whatever materials were needed in the beginning.. much more materials have to be added for the change-over.
it's fun when you can make many of a cheap kind, like farm houses.. but costly just for the fun to change model/color of a limited amount of crafting building.
this is my personal opinion.. I just see it as a waste of time, materials, energy to change aspect of a perfectly functioning building.
a third should have been offered.. then yes it would be worthwhile and speeding fun.