To list a product on eBay, it is required to have the shipping weight. In the product details, you will see a shipping information section. There you will see a "Shipping Weight Major" and "Shipping Weight Minor". 

Make sure that you have the Major AND Minor section filled out. If your item weighs less than 1 pound, put "0" in the Major section. If your item weighs an exact pound, then put a "0" in the Minor section. Once you have made these revisions, you can then successfully list your product.