You can also PM a mediator if needed

Yes,

Here is an entire list of every mediators in JSP.

You can PM an available one.

http://forums.d2jsp.org/topic.php?t=37992899&f=34

You see it when you make a thread in the trading grounds.

And no, if someone gets scammed, they made the decision not to use a mediator and have themselves to blame.
If the scammer decides to pay back, which happens often, then the scammed gets...paid back mmkay.