Just had a look at the ad' and it states that the gun will be delivered via RMSD. I'm not sure, but I'm thinking that these parcels my be sent by air and are more likely to be checked (possibly by X-ray) than normal mail. If this is the case, perhaps it is awaiting inspection at RM.
Personally, I think this is clutching at straws as the seller's not responding to your messages but it's the only hope I can offer.
I know you said that you send a tracking number when posting, did you get one from the seller?