Depending on your age, just tell him that you are a college student and that your class is having a fund raiser/contest. The person who collects the most buckets wins a trip or something. This will entice him to keep future buckets for you. If he asks what the school does with the buckets, then you could use the excuse that they sell them to contractors and that the profits goes to events for the school. Once you mention you are doing a school fundraiser and then mention you are not asking for money he would likely be happy to give you the buckets. Come up with a name for the fundraiser........................Buckets of Knowledge (or wisdom, whatever). This would likely work.