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If you determine it's not a scam (which it sounds like it might be), take only a non-refundable deposit of $500 via Paypal, to avoid their fees, then the balance via a bank wire transfer or certified bank check. Make it clear that the bike will not move one inch until all funds are received and clear your bank!
An escrow service is an alternative. I've bought and sold high ticket items long distance that way. In my transactions the buyer sends the money to escrow. Escrow verifies funds and holds the money until the buyer is satisfied with the item, and then releases the money to the seller.