I ran into this exact problem a couple days ago. The way you remove it is to replace the exterior roof, walls, windows, and door, all at the same time. Once you do that, you'll finally see a preview of the new skins. Confirming the change at this point will remove the merchant skin, which will return to your inventory, and apply the new skins. In addition, you'll be able to add the other elements, that is, fence, wall decoration, and roof ornament.