Coal and diamonds (not in ore blocks) can be sold back to the server. Diamonds buy back is $350 each. Coal is $8 each. Sorry but emeralds and obsidian can not be sold back to the server.
Ore blocks of the following can be sold back to the server:
Redstone $40 per block Stack $2560
Lapis $300 per block Stack $19200
Iron $30 per block Stack $1920
Gold $50 per block Stack $3200
To sell back to the server there are two commands:
/sell all - this will sells all ores you holding in your inventory. Be careful with this command in case you want to keep some ore blocks.
/sell hand - this only sells what is in your hand.
Anything else you can sell through /market or to other players using the trade.
Hope this helps people out.