Hmm..what's your definition of fairness?
I see that the fights start after a "Ready, Go" or any such sort from the witness..that's fairer than PK in which sometimes you are caught unawares.
As for anything goes, well, a non GM would find it harder to kill a GM, but potions / wands / scrolls / mounts et al should at least even things out, somewhat. (Ok, that's probably a blatant lie, since potions/wands weigh around 10KG and GM can cast buffs for free.)
But you can counter that. Put these items in your clan container in every room in the arena, and take them out and use when needed (assuming there's a script that prevents non clannies from picking them up.)