In the kind of case I'm thinking of, the sale has essentially been concluded online, the buyer's money deducted from his bank account, and an "auto" receipt sent, courtesy of the seller's own software. So when it comes to this:-
 
 
	 No problem with the first bit of the above - it will definitely be a genuine and honest mistake on their part, but with regard to the emboldened text, you could almost certainly argue that the buyer will have noticed, because from their perspective they've spotted a bargain. However, the contract of sale is surely complete by default, as the seller has left it to his own software to oversee the transaction? The fact he offered the coin at the wrong price, and wasn't supervising the sale itself, is down to his own carelessness, and the buyer has bought in good faith. In my opinion, any rate.