1. they are in physical possession of the money order,
2. they paid anything in value for the money order,
3. they act in good faith in paying it,
4. they did not have notice of the cancelation.
To qualify for holder in due course, they have to show that they are not negligent in paying the money order. Since they are a check cashier agency and not a bank, they have a duty to check the authenticity of the money order from someone that is not a regular customer, your ebay seller. Even if the ebay seller is their regular customer, they still have a duty to check the authenticity of the money order, since it's not an insubstantial amount. Obviously, they did not meet their duty and were negligent in paying the money order, (if they ever did pay it,) then, they are responsible for their own negligence, not you.