I think your guess of the Apmex listing being an error is correct. They could have canceled the transaction after noticing it sold at that price (although it is possible they actually mailed it out).
Just got an email survey from eBay wanting to know how satisfied I was with the transaction. The transaction was fine - but one of the followups was "how do you feel about the following statements", and those statements included "it's easy to find trustworthy sellers" and something to the effect of "eBay is a safe place to shop". I'm afraid they won't be happy with their numbers on those questions...