The following are a few basic examples of what you should include in the item description (and will not apply to every situation):
Picture - include at least one good quality and properly sized image, certain items may require far more. A picture is always worth a thousand words. When preparing your image(s), please be mindful of those on dial up.
Product Material - state what the item is made of.
Size and Weight - include size, dimensions and weight, this will ensure the item will meet your buyers needs and will also allow them to roughly verify your shipping costs if they are in doubt.
Colour - state the colour, your picture may show a different colour on some browsers.
Condition and Packaging - state if it is new in the box, new, like new, excellent, gently used, good, fair, etc. If you have the original packaging, be sure to state this.
Flaws - identify any flaws, honesty is the best policy and will reduce the risk of an unhappy buyer. Look for cracks, chips, marks, tears, discolouration, holes. Include a picture of the flaw in the listing.
Written Material - if you are including manufacturer instructions, or user manuals, etc, be sure to include this in the listing.