Marcello and his girlfriend are amazing people and amazing hosts. We discovered with Marcello's help and tips places and restaurants we would never have found otherwise. You are getting a lot more than just a place to stay, you are getting a wonderful experience.
The apartment itself is very clean, cozy, and close to the Palermo cathedral and the main attractions. Ballaro market is less than 10 minutes away, the cathedral 2 minutes away.
So go ahead, stay in Marcello's apartment. It will make your trip 10 times better.