The advertised room turned out unavailable due to an emergency and Costanza kindly offered a different accomodation: an apartment to the north of the river. That place was very good: with all the amenities, near the tram station, and not far from the center (about 20 minutes walking from the Uffitzi gallery, for example). Costanza was very responsive.
One note though - it is more about airbnb app than about the accomodation. Because Costanza communicates in Italian, the airbnb site automatically translates everything she sends to English -- including things like wifi name and password. So, for those things, please disable the translation.