Are you sure about that? I see this as a maze in the grey area of the law.
While the home is for sale, doesn't the owner still have the right to say if firearms are permitted or not?
If they still live there I think they have that choice. Kinda like smoking. They can say "no smoking" and be accepted as legal.
I imagine that if they still live in the home they can say no firearms. However if they do not live there any longer I don't think they can say that. Likewise if they have the home for sale and feel uncomfortable with firearms, they can always go to the store while you're there.
One argument I can nearly guarantee will come up though is if you feel THAT uncomfortable viewing a house for sale that you must have a firearm, don't go in. Personally, if my home is for sale and you come to look at it because you're considering buying it, I don't care if you're CCing or OCing (just no rifles or shotguns). Hell, Grapeshot if you came to my home OCing, it wouldn't bother me any. Just buy the damned place already, alright?