I'm pretty sure the previously unconfirmed Sweet Caporal 350 Factory 30 back was the reason why it went for so much. However, if that was the reason, the buyer is NOT going to be happy when they get the card. I know that the seller included the wrong back scan in the listing - the back is actually a Sweet Caporal 150.
The seller provided me with this information a couple of days ago when I asked for confirmation of the 350 series back.
Hopefully they can get a refund.
Steve