Not 100% sure how much the knife is worth, but i do know about Paypal. When being sent the payment as friends and family they can chargeback the payment by filing a "Unauthorized Transaction" suit which they just claim that someone hacked their account and sent the payment(which they can easily do by using a VPN and as soon as Paypal see's the weird IP they usually win back the money 99% of the time).
I suggest having him transfer it to Bitcoin and/or another non-reversible currency.
Good luck buddy.