It gets better... I didn't notice it before, but the actual shipping cost was $66, not the $35 I paid. No matter what this guy does, it's going to cost him. If he does nothing, he probably gets negative feedback, trashing his record; if both buyers pursue resolution via eBay, he eats $130+ in return shipping; if he gets creative, and convinces both buyers to ship directly to each other, he eats $130+ in shipping costs; if he issues a partial refund to the parties to compensate for his carelessness, he's going to eat that cost... no matter what he does, he can't win.

The moral of this story: if you ship expensive things simultaneously to different parties, make sure you send the right stuff to the right folks.

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Update: seller got back to me, wants to arrange a "swap." This will be interesting.