Your first feedback to a buyer must be positive. You can follow up with another comment which can be negative. You can also reply to their negative comment.  As space is limited, you have to play around with wording to fit what you want to say in. It should all be factual and checkable.
I used to sell 250 collectables and antiques on eBay per annum. It's changed days, and I don't know whether the dog which has had its day is me or eBay.