I have seen quite a few of the card backs around so I would say it's not rare. As you can see from that previous auction people are willing to pay silly money for it so I would say let it run on eBay if you wanted to sell it. It appears several people were bidding quite a lot to get it.
Or
You could keep it and start collecting the other 30 back cards.
Totally agree with Mark, it's not a particularly rare card, but not super common too. It certainly commands a premium though! Before the recent one on eBay I would say it is a 100/150 card.