No action in any gear probably means very low on fluid or no internal hydralic pressure. Make sure the driveshaft is not turning in gear though with a failed rear axel or rear end.

It has happened before when the owner thought the transmission was out. You should almost count on having to change the transmission out if there is enough fluid in there to be safe in the deal. It should lower the price of the coupe enough to easily compensate for it if you change it out yourself.

If this is not a negotiating tool I would not know what one was. He obviously does not want to deal with the issue. Best of luck...