Yes, ivory that is confiscated and stored by governments should be open for sale, because it would reduce the cost of the ivory from poachers who sell the ivory on the black market. Additionally, if the ivory is already harvested, it might as well be used instead of thrown away. It can raise money for law enforcement.
In principle I can see the merit in this idea, but the main problem that I see the fact that it would be relatively easy for a poacher to pass it off as legitimate business. Even if it required some government seal how difficult would it be for a corrupt official to take a bit of money to give approval? At that point you create a larger problem. Suddenly people who think they are funding conservation are really funding poaching.
Ivory that is confiscated or obtained from culls and stored by governments should not be open for sale. If it was for sale, this may encourage people to want more, thus encouraging the poaching of elephants even more. Additionally, selling the ivory only a little bit may make it more rare or a collector's item which would also increase elephant poaching.