"This card is the property of Sky Subscriber Services Ltd.
and must be returned upon request"
The card enables you to view Sky services, is still owned by them and legally not yours to sell.
That's not to say that people don't sell them but be aware that it could be de-activated at any time plus there are rumours ATM of new cards being issued.
If that's true it would render the existing card useless when/if cards are replaced