For each battle you can choose to put a wager on whom will win.
This means betting credits, gold or items of which the winner will win.
When organising a bet include what you are betting in the PM you send to a Mod. Then both players must send a trade to that Mod labelled 'Wager' or something of the like. When the trade is completed the battle can begin and the winner will receive both items, amount of gold etc.
If credits are used, there is no trading required. The amount of credits is simply removed from one players and added to anothers.
If the wager is over 1000g from each player, 1% of the gold may be taken as payment for handling the large amount of money. However this may not happen in every case.