Right or wrong, the buyer sounds like an arrogant jerk. I can't think of much I can do to help you at this point, but here are some packing tips that may save you some future grief.
One of the biggest factors, is that the loco should be packed so it can't move around inside it's box. If the loco can move around, it's like a person in an automobile in a wreck that is NOT wearing a seat belt. The car hits something solid and stops dead, and the person inside becomes a projectile and hits the windshield or whatever.
Those engines are heavy, and if not secured inside the box, they can fly forward or backward with a lot of inertia and stuff can get bent. Make sure stuff is immobilized in the center of the box.
Put that box in a larger shipping carton and pad it on all 6 sides. A couple inches of padding on all sides is generally sufficient. And yes....mark the model/models in an unobtrusive spot.
Doing it that way has worked for me many many times!
Regards and Happy Holidays,
Simon
PS: I'd block that toady from bidding on anything else!