I get the impression that the piano being "d'appoint" is to emphasise that it is little used.
I went back to re-read the advert, it says indeed that it is "très peu utilisé, that the vendeur is from a conservatoire, that the piano has been chosen for its its good tone and with much care.
As for the question Betty asks about why a "spare" piano, perhaps I should explain that OH and I have been known to argue about whose turn it is to play (yes exactly like 2 kids) and, in the past, I have always said that should we come to divorce, I wanted the cats (we had 2 now alas long since gone to catty heaven) and the piano!
Also, I believe it was Horowitz who had a piano on each flour of his 4-storey house. So you see, some people do have justification for having more than one piano!
Apprendre une langue, c'est faire un voyage différent chaque jour.
from Fle pour les curieux