Why can't you give negative feedback
Because when sellers could leave negative feedback for buyers, when they were left negative feedback themselves some would retaliate by leaving the same for their buyer.
Sellers can now cancel a sale if the buyer hasn't paid after four days, see: https://www.ebay.co.uk/help/policies/payment-policies/unpaid-item-policy?id=4271
This will immediately cancel the sale and you will receive an account credit for your selling fee. Neither party can leave feedback; any already left can be removed. The buyer will receive an unpaid item strike for not paying, which will stay on their account for a year. Two strikes mean that most sellers will automatically block them from bidding.
To add non-payers to your blocked bidders list and prevent them from sending you messages, see: https://offer.ebay.co.uk/ws/eBayISAPI.dll?BuyerBlockPreferences
