Missiles are considered weapons/hazardous materials. Even if the object is "deactivated" it can still hold explosive ordinance in it.
And missile launchers are okay as long as they don't have any weapons or stuff on it... Yeah, I think that's what they said in the Ebay moderator manual.
Eh... The ebay thing is twofold: Don't want people to be able to buy weapons from their site (and support terrorism and all that).
And they don't want to blow up some poor mailman or something. they've got enough troubles and don't need to have some nuclear warhead detonating in their truck to add onto them.
Well, yeah, I understand that. But it seems the guy was able to sell the launcher AND the missile separately. If they were concerned as you say, he should have been prevented from selling them period.