I originally bought 3 of these cards used off of eBay for a small fortune.
They have provided dependable service for 9 months. I have no problem running the gpu's at 900 mhz, I've gone up to 920 mhz but I preferred the power usage of 900 mhz. In addition these cards have the dual bios which switches the cards up in voltage and allows clocks into the 940-970 mhz range but at great increase in power usage. I have the original invoice for this card and so if for some reason the card goes belly up I can provide warranty proxy.
Here is the cgminer output of 1 card running at 920 mhz.
0: 920 800 1.120 74.5C 5371 99% | 405.4/410.6Mh/s | A:8075 R:45 HW:0 U:5.51/m I:10
1: 920 800 1.120 67.0C 30% | 419.9/410.7Mh/s | A:8143 R:43 HW:0 U:5.55/m I:10
The best I ever got was from 3 6990's in one machine pulling 2630 mhash/sec at the higher voltage.
I'm not sure what is fair pricing for a 6990 right now ... make an offer.
You can pm to sunbreak on freenode irc. Please reasonable offers only, don't waste my time. You can also just reply to this.
I'll accept BTC or MTGOX USD cash redeem code, or even PayPal if you pay the fees.
I'll be selling the 3rd card as well, but I have it loaned out right now. Should be coming back soon.