Yes selling diamonds can only be done player to player (currently)
When the server was first launched for 1.0 you could sell them, that ended shortly!
If they were to be sold to the market again the would be extremely low in value, possibly lower than iron ingots
because they are extremely renewable.
say there are 100+ voters a day thats 400+ diamonds that are put into the game effortlessly, which is highly unlikely to be achieved daily without voting.
So for now sell to other players, ask around for prices, hold an auction.
TL;DR: Sell to other players.