You should have posted this photos before. This fin box is broken, cracked and need to be repair ASAP. I am sure you have water in you board as well.
This fin box broke when the fin hit a rock or entering the shore. I am sure about that. The guy who sold you the board is a scammer. He should give you the money back.
That is all I have to say.
In your first picture you can see the box is already disconnected from the board slightly and the third picture you can see a hole inside the box side wall.