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PCJamie's deal was done as soon as he accepted the seller's offer for $300. At that point there was a binding contract. Anything after that -- a failure to pay by Jamie, or a failure to deliver by the seller -- is a breach. There is no grey area here. If Jamie were so inclined, he could force the seller to complete the transaction (as Adam pointed out), particularly since this card is unique.
And anyone emailing the seller AFTER they knew a deal was made, seeking to break the deal by making a better offer, has tortiously interfered with that contract.