Interesting idea!
What is the incentive for voter participation. Here in the US for instance, people don't particularly enjoy getting called in for jury duty.
Perhaps there could be a set-aside in the transaction in-case the transaction goes into arbitration. Those funds could be used to pay a jury for their opinions.
Potential challenge: If parties are involved in an arbitration, what is to prevent one party from setting up 100 (or 1000) pseudonyms and/or simply getting their buddies to vote their way.
It's possible that community spirit or natural tendencies of some people just "wanting to help out" would be enough to gather votes. Think about wikipedia - it is very successful, and many users put many hours in to it, despite there being no rewards in it for them. But, it's also possible that the sense of being helpful would not be sufficient, and transaction fees of some sort would have to be included to help resolve such disputes.
As I said in the OP, the plan should make attempts to negate voter fraud, by requiring accounts in order to vote, and not allowing more than one account per IP and email address. Also, not allowing public proxy IP's to register. Also, not allowing more than X registrations in one hour, where X is 1.5 times the average number of previous registrations per hour in the last 24 hours. Perhaps, even go so far as to say that only accounts that have had a successful transaction through the company would be allowed to vote on any other cases. And if two accounts suddenly push through a transaction, then immediately vote on a pending case, those votes might be removed as instances of possible voter fraud. On top of that, do not allow new accounts vote on cases that were already in process while they registered.
There's lots of ways to help negate voter fraud. The best way, however, is to just have a bunch of people using it.
I think in any case, the option should be given to an overseeing judge or panel of judges to overrule the majority vote, in cases where it looks like voter fraud may have taken place, or the vote went the opposite direction compared to obvious evidence. The judge(s) would be trusted employees of the company.
