You need Credits for trade taxes. You need a few unveiled(opened) rivens for both of us accounts safety.
For 1000 platinum you need 2 unveiled(opened) rivens and 500.000 credits.
For 2000 platinum you need 3 unveiled(opened) rivens and 1.000.000 credits.
For 3000 platinum you need 4 unveiled(opened) rivens and 1.500.000 credits.
For 4000 platinum you need 6 unveiled(opened) rivens and 2.000.000 credits.
IMPORTANT: Do not talk about platinum sell/buy in game with anyone.