do /market
click the chest in the bottom left that says "stock"
click and move your item you wish to sell onto one of the black panes
right click the item- there will be a GUI with a red pane 2 of the items you are listing and a green pane
the red pane is cancel
the first item is to raise the PRICE of the item you are listing that can be changed with 1-9 (note: numbers cannot be entered on the numpad, you must use the keys on the top of the keyboard)
1-1
2-5
3-10
4-50
5-100
6-500
7-1,000
8-5,000
9-10,000
the second item is the AMOUNT you are selling that can be changed with 1-9
1-1
2-2
3-3
4-4
5 to 9- all 64
the green pane is to confirm your listing and will post it on the /market
shift+click x2 on your listing to remove it from the /market
when you purchase an item on /market, you will not be notified with something such as "you have successfully bought 64 dirt," it goes strait to your stock chest.
learn more about /market here