That's what happens to cars. Cars get seized by authorities and then auctioned later.
If the governments turn around and sell the ivory cheaper then what the poachers would, buyers would more then likely pursue the legal routes of buying it via the governments.
When a shit ton of ivory is recovered from illegal poaching, it can then be legalized, sold way cheaper and the price of it would come down some overall, especially since there isnt much of the legal stuff out there now. If new legal ivory were introduced to the import export market, it's value would go down significantly as well as the poaching. Once the poaching drops to do less demand from the buyers getting it from the poachers then the less elephants will be killed.
Until it all starts all over again, but at least this way it buys some time and helps out countries that could put the money to good use.