Think of friction. You have static friction and dynamic friction.
Based on the risk (successful tx / total tx) for a given payment method, someone trying to trade should have in escrow:
dynamic risk = past experience
static risk = risk with reduced participation of scammers
reserve = escrow amount
reserve * dynamic risk * static risk / dynamic risk * user risk for a given rating
There are a few scenarios of scamming:
They don't actually have BTCs. Problem solved.
They don't send BTCs. Escrow awarded.
They don't actually have dollars. This is least likely because no one would send to an unrated user who doesn't put out the cash.
They don't send dollars. Chargeback immediately results in non-reversible escrow award. Never allow the chargeback until BTCs are returned.