KJJ is owned by eBay but they don't have anything like the Seller Protection you have here.
The site is best for local sales. Not mail order.
If the guy does return the shoes (or anything-- I smell a scam too) he must use Delivery Confirmation or Paypal will not refund him.
So.
He said he will return the shoes.
If he does and he is right (it could happen) or even if he just returns the shoes because he realized $400 odd dollars was outside his budget, refund him promptly.
If he does not, do not refund him.
If he starts a PP claim , don't panic. Just tell him to return the shoes for a full refund. If he does, refund him . If he does not , PP will not back his claim.
I repeat. NO REFUND WITHOUT RETURN.
Don't worry about the "fake " claim. It is not relevant to you.
KJJ is best done on a cash on the doorstep (or at Tim Hortons) basis. There is little or no Buyer or Seller Protection there. Cash on delivery. Not Paypal.