The Unique sniper rifle, The Pink Slip, is supposed to be sold by the vender in Forsythe Luxury Munitions, located in Byzantium, however the item doesn’t appear in his inventory for most players, myself included. You will see in my screenshot that he is selling one of the Unique weapons he normally would (The Executioner), but still not the Pink Slip. I have started a couple of new game files after this, and went to Byzantium to see that he wasn’t selling either of the weapons.