The ebay feedback system has this weakness. Sellers should only be able to get feedback once they give it. That's not how it works at present. Most sellers withold feedback until they get it, IMHO to be able to retaliate if they get a negative. Buyers are generally reluctant to leave negative feedback because of spiteful retaliation from the seller even if payment was prompt. Any negative feedback detracts from your score, hence the desire to avoid spiteful retaliation. I know I have given positives when the seller's service was horrible, just to avoid the retaliation.

If ebay changed the sysem, the feedback scores would be more accurate.