You must write a clear, informative and lengthy item description. Always be truthful in your description and include in your description any defects with the product. Look at one of the sample items I found on eBay in advanced eBay selling for one great and one terrible description.
2 A good pictureAs the saying goes a picture is worth a thousand words and that is certainly true with selling online because the customer can no longer feel or see the item themselves so they need your picture and description to replace that experience.
3 Reasonable shippingThe thing buyers hate most is finding a great deal but realizing shipping is going to cost them an arm and a leg. I find the most successful listings for the buyer and the seller charge for the full cost of the shipping + the packaging materials. The other option is to give free shipping as eBay favours this but only do so if you can afford this and still have a healthy profit margin. You could always include some of the cost of the shipping in the item price and then offer free shipping.
4 PriceYes I rate price at number 4 because contrary to what you have probably heard is that price isn’t that important. of course you need to be competitive but if you look professional, take a good picture, describe your item well and charge reasonably for shipping then you can often sell for far more than the other sellers. This is because people still aren’t that confident buying online and when you promise to offer a professional service and back that up by looking professional then people are willing to pay more for that confidence.