Anyone can be an arbitrator, you just tick, 'willing to arbitrate' on your profile and fill in whatever info you want on your profile about what sort of contracts you will arbitrate and what you will charge. You can charge a fixed price, a percentage or nothing.

 Everybody needs to check that the arbitrator is who they say they are from some other source such as this forum or a known website or blog where they can publish their crowex user name. Both parties need to trust the arbitrator.

 The arbitrator cannot ever access the BTC he/she can only choose who gets them. The arbitrator is not even contacted unless there is a dispute and doesn't know anything about the crowex address or the contract unless a problem arises. Then if someone starts ta dispute he arbitrator waits to get any fees required off the two parties before making a decision. The arbitrator can join in the chat box and chat to both parties about the contract.

 I just asked fuzzyBear from this forum to do the arbitration for these bets because he is independent and trustworthy.