Assuming you made clear that it was a used item, then it's acknowledged by reasonable people that there's always some risk with buying secondhand. He took that risk, and unfortunately he came off worst.
Be honest, if it were you, you'd be annoyed, wouldn't you? And try to get a refund?
If you didn't offer a warranty or return, then you should be OK. I say 'should' because as others have said, Paypal tend to be heavily biassed in favour of the buyer.
Let him initiate the legal action. As you have the positive feedback, indicating he received it, and was happy at the time, you have a strong defence.
And shineybomb, when I sell, I don't offer the option of NOT having recorded delivery!