Latest email from him, sounds shady:
I'm willing to go further with the deal but have to finish installing the new clutch and flywheel set. I only have limited time and opportunity to work on the car because of excessive work hours. I have the mustang over a friends house in his pole barn where we work on it when we have time. I got a strange vibration after a launch and initially thought it was the harmonic balancer. We installed a new balancer and still got a vibration. We then examined the car and discovered a cracked bell housing so we assumed this was causing the vibration. We swapped a good bell on the trans, bolted everything back up (without removing clutch and flywheel). To our surprise, the vibration was still present.
So now we've come to the conclusion that the vibration must be between the pressure plate and flywheel. Lesson learned to check everything while trans is out.
We've logically narrowed it down to clutch assembly and or flywheel based on parts replaced and the fact that the vibration is felt while standing still eliminating any other component. We've theorized one of the fingers in the pressure plate could be lose, dislodged spring on clutch disk or dislodged or loose flywheel weight. So back under the car I will have to go to remove the clutch assembly, inspect and replace any parts that's out of place. My biggest regret was not removing clutch assembly the first time and assuming all was good with the replaced bell.
I know this could sound scary but when it's all said and done, the mustang would have a new harmonic balancer, replaced bell, new seals on trans, new rear main seal and what ever needs replacing in the clutch assembly