Assuming that is for protection, assuming postal inspectors, etc will come down hard on them for defrauding the govt, I wouldnt count in it. They are too busy with everything else to worry about your $500 loss.
Local sales only, cash in hand, or if you MUST cross state lines use some kind of escrow service to protect your money.
Non face to face transactions just arent worth it anymore, IMHO. Risk is too high due to scammers.
Postal money orders for gun broker if they don't take credit. I've had people back out of deals on tools when I said I'd only pay with pmo, so it still carries weight vs anything else.