Firstly, he's not required to do what I suggested, it's only given as personal advice which he was soliciting...
Next, he actually wouldn't be out of a knife, since it got returned to him.
And Last, if the guy destroyed it before returning it...just how far do you think you're gonna get trying to deal with him further?
You're gonna demand he keep the knife and you ship it back to him-- now you're REALLY out a knife because he's gonna keep it this time and STILL demand a refund and PayPal's probably gonna side with him cause he's the buyer. Now you're in sh*t slingin match with a pig <you know how pigs love sh*t, right?> who's dishonest to begin with and you're stuck with the burden of proof...