How do I list an item for sale on amazon that isnt already on amazon?

What you could do is search it on Amazon and see if another seller is selling the item. If another seller is selling the item, there should be a button that says ';Have one to sell'; click on that and you should be in business. If you cannot find any listing for the item that you want to sell, then you are out of luck. Your other option if you can't list it on Amazon, list it on eBay.