No way is perfect. MoneyPak has it's usefulness in our marketplace.
Cash is great, but not convenient. You will have to meet up with someone. You can always get robbed, too.
Western Union is good but the fees can be very high depending where funds are being sent. Plus, most sellers will want you to get off of your ass and go send cash at a location. So there goes your convenience.
Bank Transfers...like an online bank transfer? Those can be reversed fairly easily from what I hear. A wire is probably better, but high fees are usually associated with wires.
MoneyPak reversals do happen, but with proper risk mitigation MP's are a pretty handy selling method. What you really have to watch out for is GreenDot shutting you down for too many/multi-location loads. Paypal may shut you down as well.