I think void should be tradeable mainly because it is a reward that you work hard for but may not have the requirements to wear or don't ant the requirements to wear it, you can't sell it.....
If i'm not mistaken u don't lose it at death...
So it's a good reward to use when pking....
[edit] i will make it so that if an untradable item is dropped in pvp, it'll drop a coin reward;)
Bad idea, people can kill each other for more coins 
No reward is given for pvpfarming;)