The conclusion at first:
We were fully pleased with the apartment and Marco's hospitality.
The apartment is located in the center of Naples walking distance from the train station (but far enough from that :) , the ferry stations and it is at the edge of the historical downtown. Almost all the main attractions can be reached on feet easily. The apartment is neat and tidy, very clean ( actually spotless) and just as stylish and cosy as it is on the photos. It is on the 3rd floor without elevator so some stairs climbing is needed but the quality and the location of the flat are just worth that. Night time the place is quiet however daytime the close main road traffic can be heard but that is not unbearable.
You can find not just the needed basic equipment (cutleries, crockeries, etc.) but so many more thing (iron board, mosquito killer, fresheners, pasta, spices, coffee, sugar, maps, travel books and so on) ... you won't miss anything for your stay.
There is a pizzeria/restaurant and a cafe in the building and trattoria (family restaurant) in the street. All are recommended.
The check-in and check-out went smooth and flawless due to the great communication with Marco. His fast reaction time to our questions made our trip easy and stress-free.
We could arrange everything in prior of our arrival with him and Marco waited us in front of the building and helped to get our luggage onto the 3rd floor. He explained everything we needed to know regarding the flat and about the city, he gave us tips for travel, for eating and answered our several questions.
At check-out he was at the place in time (as we agreed in advance) and arranged a lift for us to the train station.
Actually Marco is a natural born host, an open minded, very friendly person. He continually took care of us not just during our stay in Naples but after that when we needed advice in Sorrento :) He usually asked about our day and if we needed any help.
You can ask him anything, any kind of help even in prior of your travel. Extraordinary.
So the apartment and its location are worth the renting but Marco's attitude and amazing hospitality made us totally confident not to hesitate to rent this apartment if we visit Naples again and we recommend that for everyone who wish for a problem-free and comfortable place to stay.
@Marco, Grazie mille per tutto! Respect for all the help you did ;)