As the title states. Allow the vendors to offer a small amount of very high end items that they cant stock regularly but can source for the player upon request.
Example:
Say I want to obtain a Seekins SP10M (i know its not in game which is why im using it as an example). I go to Gunny and he will have a dialogue menu or other means of communicating this and he will get one but it will be very expensive and take a while to get.
This will add more options for end game equipment while also maintaining their value because A) they cant be looted thru crates B) losing one to death should be painful.