How do I list a custom item for sale?

When you go to the "Sell" tab, you must first select which kind of custom item you are adding to your inventory (set, part, minifigure, etc). On the next page, you add all of the pertinent information about the custom item and then select "Custom Item" below the Set Number. Please see screenshot below for reference:
If you cannot find the right category for your custom item, you can simply choose "Other".