It's almost an art to write an advertisement for ebay listings. Put yourself in the buyers' shoes. Without being able to physically inspect the goods, a good description and pictures are invaluable.
Pictures are the most important. A picture tells a thousand words. It can tell the quality of the item, the shape, size. So make sure you have a picture. Sometimes, you may need more than 1 picture to ensure you can show all angles, or how the product works. Make sure you present the picture well, eg. crop unrelated parts of the picture, make sure the picture is not upside down, make sure the background is not cluttered etc
Description is the trickiest bit. Make sure you describe the product to the best you can. Include the colour, measurements, what it is used for, condition of the product. If the product is a technical item, you could reference it to the manufacturers' website. Sometimes, if you know the market value of the goods, put that in your listing! The market value helps ebay buyers gauge how much is fair to pay for the item. Sometimes, it helps too if you explain why you are selling it eg. unwanted gift, moving house etc. Buyers will smell a rat if the item is perfect but you are selling it. In your description, if possible, offer postage costs so buyers can estimate the total cost. A better description prevents you from answering many potential buyers' questions. With the right description, you can sell anything and everything on ebay. Good luck with selling on ebay!