Thanks all for the advice.
In an unexpectedly positive turn of events the seller did respond to my message overnight. Unfortunately it was in fairly poor English, but, from the gist of it he was saying it hasn't gone anywhere because it is Sunday. Figuring I was now getting the run around I fairly strongly pointed out that I purchased the card over two weeks ago and there had been plenty of other days it could have been picked up.
Low and behold when I got home from work I had an update to the card's tracking with it now showing as being with USPS.
I will (obviously) keep monitoring it more closely but may have been a genuine mistake by the seller.