The real estate agents put up signs that can be seen from the street in the windows of the apartments that are for rent/sale. I do not know how they handle the apartments that can not be seen from the street. The real estate agents advertise the available apartments that are for rent/sale with photos in the windows of their offices. The Clarin Newspaper lists apartments for rent/sale in the classifieds and they are organized according to Barrio.
Hope this helps
Most signs you see in Buenos Aires are baits and switches put up by real estate agents to advertise . I do not agree with this as it looks ugly. The worst offenders are a few well known agencies in Palermo Soho and San Telmo who have monolopised the market .