EBay does not use feedback to assess member accounts. That is considered an expression of buyer opinion. Some buyers do use it to assess the reliability of sellers, so responses to negative or neutral feedback should be brief, factual, polite and very very rare. Let the nutbars expose themselves, you don't have to.
Satisfaction Guaranteed or your Money Refunded has been a standard of commerce since the 19th century. (That specific wording was used by Timothy Eaton from 1869 https://www.cbc.ca/archives/entry/eatons-satisfaction-guaranteed )
If you are having problems making a profit, look to your business plan.
Are you charging too little for your service/product?
Surprisingly often, a higher price leads to greater customer satisfaction.
And bottom feeder clients are notoriously unhappy with their cheap goods-- all too often suspicious that their bargain was flawed in some way which has nothing to do with the seller or the product.
