If it's a survival server I wouldn't recommend to sell them for diamonds or other ores because some people might use xray, so I guess it would be better to sell them for server currency. But if the server is only with players that you trust and you know that wont xray then it would't be a bad idea if you were selling them for diamond or in game ores. Now for the prices it depends, if its easy to make in game money (server's currency) then the items should be sell for more money.
I hope that with this I helped you :)