Current rates for used cards on average are as follows.
5830 - 6.5btc shiped USPS
5870 - 8btc
6990 - 12-14btc depending on colling
numbers are bassed on what I have payed and seen in the forums. Good luck with your sale
Sign me up at those rates, I'll take all of the 6990s you have at 14BTC, heck I'll even jump above what you are asking.