Russ
Since you have not been paid for the aircraft you do not have a contract (no consideration no contract). You were wise indeed to send scans of the logs. Take control of the situation by simply sending him an email(return receipt) or letter (registered) stating that if the sale is not complete by mm/dd/yyyy you will conclude that he is not interested in completing the purchase. Include copies of the billings of the pre-buys which he agreed to pay and an acounting of his net refund. If you don't hear from him or he balks send his refund the day after the aforementiond date.
Coming to an agreement is a two way street and all parties should come to a final decision with confidence. I believe a purchaser should always choose the person who does the pre-purchase and in this case I hope this was so.