He doesn't have a leg to stand on, you didn't sell it with a warranty, mechanical things break and its a fact of life. If he bought it when it was working fine and now it's not working, he's broken it and he's probably trying to pull a fast one so he doesn't have to pay to fix it.
The police can't do anything, he's trying to scare you and a 30 day money back guarantee on faulty goods would be if the goods were faulty as they arrived, that's not the same as a 30 day warranty which he I don't think you sold with it.