We are all members here like yourself, but can try to explain why it's an eBay policy violation to leave a negative feedback for a buyer, hence a buyer can only have 100% good feedback.
The leaving of negative feedback was banned many years ago, because some sellers were abusing the feedback system. For example, some were holding back items until a buyer left good feedback for them.
eBay have excellent tools for dealing with non payers. Cases for unpaid items. This gives the buyers a strike, if they gain another they cannot buy from the majority of sellers on eBay for a full 12 months. Gain more? eBay remove them permanently from their site.
Cases are very effective.
Even if a negative feedback was allowed, most sellers sell items in a Buy it Now format, so there would be not time to read any well meaning 'warning' feedbacks. Far better to open those cases, which will in little time remove a member for life from the site, if they continue to buy and not pay.
