I am fascinated that severally of you actually believe that Merry or the arbitrator could have any liability for what the defendants do. What is your theory of liability? What is your cause of action? Certainly, there is no proximate cause. Respondeat superior doesn't apply to the arbitrator nor to Merry. Negligence does not apply because there is no duty to protect others. If you kill somebody while you are hijacking their car, you are responsible for the consequences of your act; not your parents, teachers or minister; just you.
In terms of morality, I tend to agree with you. However, anyone can sue anyone else and the arbitrator decides. To say that neither Merry nor the current arbitrator would be sued on it implies a remarkable unanimous view among members of a particular AnCap society.
I imagine it as less likely the lawsuit would succeed than that it might be created. You make a good case that it is not their responsibility to protect anybody. But I am not certain that every arbitrator would agree with you.
Try a rather different example. You are in your ship heading directly toward Ceres at rather high relative velocity, and you throw out some junk you don't need. Later you decelerate and dock. Your fast-moving junk damages a docked ship and some machinery on the surface of Ceres etc. It's clear where it came from. Does anybody have a case against you? I would say yes. You are endangering people and their property. They have a right to expect you not to drop junk on them, when there was a reasonable chance it would hurt somebody. On the other hand, suppose you were in an orbit where you had no reason to think it would hurt anybody and 15 years later it did. In that case I'd say your liability is much less.
Now one that's a big closer. A man is accused of shooting somebody at random. The wounded victim testifies what he says happened. The attacker says something incoherent, the jews and the muslims are after him, he has to wait until the mother ship beams him up, live free or die, he needs some drugs to stop the mind control rays. You are the arbitrator. The victim wants money, and the attacker has it. You let him go and give him back his guns. Right after he's free, right outside the door, he sees a woman wearing a star of david necklace and shoots her. She survives, but what with one thing and another he does not.
Do you have any liability in that case? It wasn't your responsibility to pay for his medical treatment, which might anyway be ineffective. It isn't your responsibility to lock him up, either at your own expense or to make him pay for it or work for it. You aren't your brother's keeper. Why should you do anything other than see that his fine is paid and give him back his gun? Well, I think you ought to try to do something more. Here's somebody who really can't take care of himself and who really isn't up to moral responsibility. I expect if the woman who has been shot would feel it would have been good if you had arranged some better result. But she might likely not sue you over it, and she might likely lose if she does sue. You might feel as I do that there is a moral responsibility, but that doesn't have to translate into an arbitrated penalty for failing to come up with a good solution.
Arbitrators Guild? What possible legitimate purpose could such an attempt to restrain free trade, serve? In a free society, anyone can serve as an arbitrator if the parties agree. And since there is no government to enforce such a monopoly, a guild would provide no benefit to arbitrators or their clients. An aside. Even though lawyers are part of a government enforced cartel, they have no qualms about eating other lawyers alive in legal malpractice suits or other causes of action. You must be thinking of the government law enforcement guilds that close ranks to protect bad cops.
These things happen. In a world where anybody has the legal right to be an arbitrator, there could still be people who get a reputation for being good, to the point that they can charge professional prices and perhaps specialize in doing arbitration for a living. They will pay attention to each other, because they will see a lot of each other etc. They might easily have an unofficial club that only successful arbitrators get invited to join.
My father was a dentist. He did very good work and kept his prices relatively low, and he built a thriving practice. Once he organized a meeting of all the dentists in a seven-county area, not through the ADA but just a social meeting. They discussed fixing prices and made some progress, but he said there was one dentist he needed to have there who just didn't show up. The man like him had a great reputation and kept his prices low, and they really wanted him to participate. This sort of thing would surely happen even if dentists were not licensed by the state. Of course it takes a government to enforce it if it's going to be perfectly enforced, or at least enforced as perfectly as government actually enforces things..
(Grammar gripe. The preferred spelling (i.e.,used by most educated users) is "judgment." I note that most of you get it right.)
I don't like that spelling. "Judgement" is an acceptable secondary spelling and the more it gets used the closer it will come to being declared the preferred spelling.