I've been a seller on Ebay for three years and I cannot stress the importance of keeping your feedback rating clean.  It is one of the keys to your success.

When dealers and professional Ebayers are looking for a good auction, they'll tell you that it is imperative to check a sellers' feedback history.  If the seller has been given negative feedback within the past six months, they know to check the feedback to see what the problem was.  If there are several negatives, then they know this person is not one whom they wish to deal with no matter how good their prices.

First of all, keep in mind that the customer is ALWAYS right.  Sometimes this is not easy, but it can be done.  Secondly, always watch your wording when responding to an accusation.  Keep kindness the rule of your tongue!  Leave out all digs, remarks, and accusations out of your responses.  Instead, respond by keeping to the facts and apologizing, taking the blame for the unsatisfactory transaction.  Also, ask how you can remedy the situation and be prepared to follow through if your buyer wants compensation.