Here are the steps to create a Auction (sell items)

Step 1:
Type "/ah"

Step 2:
Click "Manage auctions"

Step 3:
Click a item in your inventory that you want to sell

Step 4:
You can set it to bin or bid

What is bin? If you set it in bin players will be able to buy the item your selling with a specific amount of money you will put
What is bid? If you set it in bid, players will be able to bid on your item and the player who puts more money into the item gets the item

Step 5:
Enter a item price or a starting bid depending if you're selling it in bid or bin

(Optional step) Step 6
Click on "Duration" to change the duration

Step 7:
Click "Create Auction"