Yep, if you watch new BIN listings coming up on eBay, sometimes you'll see one that's way underpriced. Sometimes the seller meant to list it as a starting bid instead of BIN, sometimes it's a typo, sometimes it's a price meant for a different listing, and sometimes it's the honest asking price. From the date of the auction and the phrasing of your post, I assume this is just a listing you found in a search, and not one you won?
Sadly I wasn't the one who bought it. I was doing some searching to find the market value of this coin (I have a MS one). My jaw dropped when I saw this sold listing. I'm not sure how often it happens but yeah a BIN monitoring system would be a great idea.
It's a little mysterious. The "buyer" left positive feedback, but the listing says "ended by seller because the item was sold". If it had simply sold at the BIN price, it would just say "listing has ended".
I would be surprised if he did. I have had a few conversations with this seller...nice man, but he's not going to let this one go for $6.