I quickly glanced at the other forum and saw where you have contacted the ATF and they are calling the dealer. I hope this resolves it.
I also see from your original post that the buyer knew you were not using a FFL dealer on your end. Unfortunately, it appears he didn't tell his dealer and his dealer is a jerk. I suppose the lesson to be learned here is that if a seller ships directly to the FFL, he should contact the dealer himself to confirm the dealer will accept it from a non-FFL.
I also see from your original post that the buyer knew you were not using a FFL dealer on your end. Unfortunately, it appears he didn't tell his dealer and his dealer is a jerk. I suppose the lesson to be learned here is that if a seller ships directly to the FFL, he should contact the dealer himself to confirm the dealer will accept it from a non-FFL.