I mentioned this somewhere, not sure if it was this thread though. Gambling should be left to PvP. Have a quest to fight a random classic build from D2 like a hammerdin, Orb Sorc, ect. After which you can open the arena you fought in for PvP combat.
By doing this it would eliminate the Pkers in lvling and questing games, make room for other games in the selection of games area, as well as have a standard area for PvP combat.
The PvP games would be broken down into catagories. one on one, team vs team, winner take all (looser is stripped of gear, kind of like HC), Free for all, Ring out, and King of the Hill (all players vs one player or NPC CHaracter)
You could also have a enviornment changes. Example could be, random curses every so often, poison clouds in areas, negative effects on skills, (maybe -5 to all skills and stats)
Instead of having chatrooms, you could have a seperate section, where anyone could watch a battle while they chat and gamble for the winner. This could be anything from gold, gems, runes, gear, ect.
I think since gold has no real purpose, this would be very useful for such a problem.
Also as a added bonus their could be a monthy torney for special unique items for the winners, a ranking for each class, and overall winner. Maybe even a super unique boss every once and a while to make things interesting, that could interfer with a battle.
On the other hand, if you want to gamble with vendors, I think their should be certain items only able to get off of the venders by gambling. Just so it has a real purpose.