It has buyer and sellers protection so there is no charge backs. PlayerAuction plays as the MM in the trade. Buyer gives cash to PlayerAuction, seller gives buyer the GP. Both traders then must agree that the trade went well. If the buyer disagrees that you did not deliver the GP, you can provide proof of in-game trade. When you do, PlayerAuction will deny the buyers claims and will give you the money.
Always take in-game trade for proof. If the buyer claims you did not deliver and you cannot proof it, then you lose the GP and money you were suppose to get.
I believe there is a firm $2 fee per trade.