Also, if they are bona fide holder in due course, it does not matter whether you canceled the money order already or notified the ebay seller, you are liable for the payment.
However, if they have not paid for and obtained the money order and are just checking with you whether the money order is authentic, you should tell them right away the money order is no longer good. But based on your previous email, I don't think this is the case.
So it's neither necessary nor helpful if you call Dallas about your cancelation now if they are already in physical possession of the money order.