Idealista

Idealista is a real estate sales and rental essentially used by agnets to advertise. I don't think any of their rentals are meant to be short term like a vacation short term. If you contact them about a listing I'm sure you'll get referred to a listing real estate or rental agent. If that's what you want then they're legit. If you're looking for an AirBnB or VRBO like service they are not that.

There are also immobiliare.it and casa.it (more for Italians) if that is what you are looking for.

Hope that helps,
=Tod

Idealista is a real estate platform born in Spain but operating in Italy and Portugal as well.
It is similar to Zillow and and RedFin in the US.
I suggested it to my brother in law and another friend to move to Portugal, and they used it successfully to find property there. If you are interested to buy property or rent long term www.idealista.it and also the Italian platform www.immobiliare.it work.
If you are interested to rent short term, defined by Italian law as less than one month, then you are better off with other platforms, like Airbnb or VRBO or Booking, or the aggregator HomeToGo.