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Sounds like a straight up con.
Emphasise to paypal buyer has already admitted materially altering the bike by stripping it.
Emphasise also that he has submitno images or proof of danage to bike or packing and that he had it for over three weeks before claiming that it was.
State that you offered a return in its original condition, but buyer has not cooperated and is demanding customs charges paid to him before even receiving the bike back...
Have you kept correspondence within eBay/Paypal messaging?