Apparently I have too many platinum:
You can buy most of the daily offers on the daily routines page for not only ancient stones, but for platinum as well. You can earn platinum here, by selling specific items. The soul-huckster is willing to give you 1-1 platinum for the following items:
1 berserker potion, 1 protection potion, 1420 soul-diamonds, 21 helm shards, 2 pet food, 0.5 minion food.
The platinum is a limited commodity, therefore you can possess only 650 platinum at once. But if you spend it, you can get more. It is NOT POSSIBLE to exchange platinums back to the original items!
You have 863 platinum (and then it will not let me use them for some reason)
Exhange 4 platinum at the soul-huckster!