second hand is fine but you need to make sure it has everything. Check the pump and the impeller (the bottom of the skimmer there is this door type thing, on the sides of the bottom there are two clips, push the clips back and pull down the door. THeres the pump) check if the pump output is connected to this green pipe, and make sure the green pipe has a silicone tube that connects to the screw at the skimmers neck. Without that silicone tube it wont produce bubbles. Dont panic if the collection cup is loose, its meant to be loose. And you dont have to have your water at that level, that level is more of a guideline, the US models should have the water level above the line on the skimmer since the US pumps are stronger, hence the question do you live in the US
There should also be an assortment of ways to hang the skimmer in the tank, check out the online manual to see them and make sure he has those parts. There should also be cleaning tools that come with it, if he doesnt have cleaning brushes its not the end of the world but first hand ones come with cleaning brushes.