Like said above plain laziness.. I am a powerseller on ebay and leave feedback once someone pays for an item.. A lot of sellers will wait until the buyer leaves feedback so in case it's a negative they can retaliate... I find this really petty.. Some sellers will do all there feedback once a week or month especially if they have lots of auctions.. The best is to email them directly and not through the ebay email service and just tell them you are trying to build up your feedback, most sellers will repsond..