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!
N'allez pas trop vite - Proust