If his story had been that he got the top on Craigslist that would one thing, but if he said it came on a vehicle he bought, then you can verify that with the vehicle history. Find out who owned before this guy. You may find out there was not recent sale, so the current owner is a liar. There may be a seller who sold it (my guess) with a soft top. So now you know current owner is a liar.
Tell him you've checked, he's lying, and you want the top back or you go to the police.