To earn gold and get the items you want you can trade other players.
To do this you can either use the Grand Exchange or trade others.
How to TradeEdit
To trade another player you right click them while crouching (by default, right click). When having the trading GUI open, be sure to check the top for which side your name is on, so you aren't placing the items in the wrong slot.
To use the Grand Exchange you right click Auctioneer James, Auctioneer Andrew, Auctioneer Augustus, or Auctioneer James. Yes, there are 2 James'.
You can also use the /market listings command to see what other players have for sale.
To place an item in the Grand Exchange you can use /market create [price] [amount] while holding the object you are trying to sell. You can also talk to 1 of the 4 auctioneers and drag an item into the hopper where you will be prompted by a sign to select the price and the amount you'd like to sell.