As Integrity stated, the most common scam is a buyer pays, then the seller leaves and never delivers the item.
The MM system prevents this because the seller must deliver the item or account to the MM first. The MM will then update and secure the account as best as possible. Then the buyer pays the seller and receives the item/account.
There is no way to prevent a payment chargeback for most payment methods, so we do not offer that type of protection.
For bitcoin, which can't be chargeback, we do offer a cheaper escrow service. For this service we hold the payment while the seller delivers the item to the buyer.