I fully agree with and appreciate Ebay's policy on disallowing negative feedback for buyers.
A buyer'responsibility is prompt and timely payment for the product. Once he or she has done that then it is the seller's job and responsibility to ensure that a good and genuine product has been delivered within the specified time. So, once a buyer makes the payment promptly, he has done his part and it is unfair for him to get a negative feedback after that.
Secondly, dear seller, you seem to have forgotten the basic principle that it is the seller who is here to offer the service and that the buyer is the consumer. THE BUYER PAYS AND THE SELLER MAKES MONEY. So, it is absurd for wanting to leave negative feedback to buyers because it is your primary responsibility as a seller to have a happy and satisfied customer.
Thirdly, you seem to forget that this is a highly competitive world of selling and there may be hundreds of sellers selling the same product and the buyer is at liberty to chose whatever he wants and if you don't offer the best, he will just hop off to other sellers. So, instead of cribbing about wanting to leave negative feedback for buyers, you try to improve the quality of you customer service and your confidence as a seller. If you sell genuine products and offer good customer service, then any buyer will be happy to leave positive feedback and you don't have to be scared of getting negative feedback.