He can most definitely leave you a negative feedback (and provide you with a seller defect). I would caution against "blocking" him prior to an amicable solution to the problem. There were a couple of times that I actually lost money on a transaction to make sure that the customer was satisfied. When you list and sell an item, you enter into an unwritten contract to provide the item to the buyer. If you fail to comply (regardless of the reason), you are subject to repercussions.