I'm pretty sure they sent out an email saying that using cards that are obtained in fraudulent ways (such as farming self-referrals) "is a potential criminal offence and could be passed to the authorities". Maybe that's why you're selling it at such a huge discount. To get the (illegally obtained) card off your hands as soon as possible.
Oh, I've done this legit. (If you count paying different people 50 cents each for their $2 referral )
I've had these for the months, and they haven't been revoked. I have friends (who are on this forum, mind you) who also farmed cards but using fraudulent referrals, and got their IPs banned and their cards revoked. No revoked cards on me.