I asked the moderator where to find a 3rd party mediator...
Did you ask it to the forum moderator? They will never involve in such kind of activities. The forum does not moderate any scam.
It's hard to understand who is right but betting is not hobby for you. If it was then you would never bother to find this place and create an account to complain against any sportsbook.
I don't understand your logic. 500$ is a considerable amount of money for me. I don't understand why I didn't try to get that money back if I think it was honestly earned even if it was a hobby.
There are no problem to get back honestly earned money but its wrong to say betting is a hobby when it's clearly does not look it's a hobby for you.