This is not very reassuring, nor would it want me to be comfortable trusting my hard-earned money at the marketplace.
If an arbitrator must be "voted in" by existing arbitrators then someone could potentially buy up a lot of arbitrator accounts to create additional arbitrator accounts which would further the attacker's ability to scam.
Seems pretty far stretched IMO. Keep in mind there will be a reputation system for arbitrator. Buying a lot of good reputation to make scams wouldn't last very long before any body figure it out and all reputations of these account will be affected. That would be a very costly and risky scam operation.