Feedback is the single most important thing on ebay. So try not to recieve any negative feedbacks for your product. When buyers see 100's of positive feedbacks for your item it gives them confidence that your product is worth the money.
Always view peoples feedback before purchasing form them. If you ever see more than 3 negative feedback saying item paid for however goods not received then do NOT buy from them. The chances are that you will probably be wasting your money, as they will not send you the item you paid for.