What do you think of this Ebay buyer?

Of course they are trying to pull a fast one! You shipped the item with tracking and USPS says that it is delivered. You can do no more. To protect yourself, attack being the best form of defence, I would advise e-bay of the situation and ask their advice. USPS will back you in the event of a dispute. E-Bay will be as concerned about this situation as you are and I am sure have dealt with similar in the past.