I'm located in Nevada. I just sold my Jeep Wrangler over the weekend and received a Certified Bank Check from the Navy Federal Credit Union in the amount of $7000. The sale was private, unwaranteed, and "as is". The car was in great working order when it was sold and the buyer even took it for a lengthy test drive. He is now claiming that it broke down on his way back to California with it, citing a possible transmission problem that was not apparent when the vehicle was sold, nor when it was inspected by my mechanic prior to my listing it for sale. He is now threatening to put a stop payment on the check if I refuse to pay for towing and repairs. Can he do this? I have a signed bill of sale stating all of the above and the title was signed over to him at the completion of the purchase. I was under the impression that a Certified Bank Check could not be cancelled and was as good as cash. What should I do next?