Remember the student in school who complained (or even cried) because they only got a ninety-nine percent mark on an assignment or test. Meanwhile, some of us were relieved (if not ecstatic) to have merely passed.
I actually witnessed one case where the math student insisted that he was not wrong. On rechecking the test, the teacher realized the answer sheet was wrong and not the student. The student got one hundred percent correct.
Realistically, however, most of us lesser mortals can't expect to get a perfect mark or score every time.
First of all, none of us are perfect. We all make mistakes.
Secondly, unlike math, some subjects are more subjective in nature. In certain subjects, the answer to a question may not be simply right or wrong. The teacher may have more latitude in how your grades are calculated. The teacher may even (consciously or subconsciously) increase or decrease your grade based on whether they like you (or your attitude) or not.
Similarly, in business, you can't please everybody all of the time.
In eBay or other Internet auctions, you will not be able to satisfy everyone. Even if you have one hundred percent positive feedback on eBay now, eventually it must fall.
Not all of your customers will be happy with you all of the time. You might make a mistake resulting in an unhappy customer. Sometimes factors beyond your control (such as a lost shipment) may result in a negative feedback.
Sometimes, a customer has a legitimate complaint. Other times the customer has jumped the gun and hasn't given you a fair chance to resolve matters. Don't let your pride stand in the way of trying to make peace with the customer to avoid or reverse any negative feedback.
Occasionally, you might even get a customer who is unreasonable. It is even possible that your buyer is a sadist who takes delight in tormenting others. Since eBay no longer allows sellers to give buyers negative feedback, to resolve any such issue, you may need to report the problem to eBay.
Even if your efforts to remove negative feedback are not successful, at least you tried. Leave a respectful comment on the matter so others can read your side of the story. Then, let your prospective customers decide if they want to do business with you or not.
I personally like it when I see an eBay seller with a perfect feedback rating. Realistically, however, I realize that you simply can't please everyone. Even if you did everything perfectly, there are still people who are just not reasonable.
Thus, even if a seller has some negative feedback, if the overwhelming majority are pleased with him, I tend to discount the few dissidents. I suggest you do the same in evaluating others as well as yourself.