I had people reverse the charge through the credit card company going over Paypal's authority long after the 60 day return policy. They even kept the item. You can argue and Paypal said they would help.
That being said. The "my agent (shipper) will pick it up" was often associated with scams. I had a guy (agent) come pick a bunch of stuff up off my trailer onto his trailer. Cash deals only. Why can't they pay cash in person? Have you spoken to a real person? Is it all email? Phone numbers?
Using Paypal and picking up in person is not uncommon. I have done it.
My guess is the shipper can be a bad actor and "steal" the stuff. Buyer receives no goods and reverses the charges for non-delivery. So you had better handle the insured shipping yourself or find out all the info about the shipper. Good luck finding the shipper even if you ID him. Calling the authorities is always fun. Court is fun too.
If the buyer has thousands in a Paypal, then he can turn that into cash...or he gets no trikes...
You could also make them use the Paypal donation feature, if they dont donate, then just send the money back(you might incur a fee).