You'll need:
What you say about your item is all a potential buyer has to go on, along with the photographs you provide. Your ad could also appear in a search listing with other items exactly the same as your own - so you need to get as much positive information in as possible. But keep it brief and to the point. Don't forget the price.
A picture is worth a thousand words. Prepare the item properly and take several good pictures of it, highlighting key features. Potential buyers are much more likely to show interest if they can see what they could buy.
Be honest if the item is damaged in any way. If you're not explicit and honest you could waste time with fruitless enquiries and/or visits.
If you have a good reason for selling the item, for example, you're upgrading to a newer model, include this in the advert.
next: Preparing Your Item for Sale