I've done a MO with a seller on GB, but only one that had solid ratings and was 'verified'. I know that's not a guarantee, but the ratings and track record do separate them from the true fraudsters. As noted, there is almost no protection with a MO, just like a personal check, cashier's check, or cash. Only a credit card offers buyer protection (well, possibly PayPal, but that's not for firearms stuff).
So, I pay Visa whenever possible, MO if the seller has a great track record, or cash face-to-face.
BB