It's not easy. Most manufacturers won't give warranty to the 2nd owner or if the cards have more than 2 years. XFX had lifetime in some items.
I suggest you don't go "all in" on 6950's anyway. I have an ASUS 6950 doing great, while I managed to burn a Gigabyte. You will burn the Rams or VRMs in the Summer operating at 550Kh/s which requires 1.175V and ~1Ghz core. In practice they only reach ~500 Kh/s at 915Mhz, 1.1V and are power hungry (almost twice) when compared with new R9-270's like the Gigabyte R9-270 OC or Sapphires. That means more cost on PSUs and power each month. The R9-280 and R7-265 are coming, so are new Nvidias that may suck less for mining. Even if they aren't the best, this alleviates the price of ATI's somewhat.
Thanks I like your reply very much but this is the most important question:
Because of the fact that they cant know you bought it used what should i ask the used card seller for so i can "prove" the company I owned it all the time? (receipt or what?)
Yes, get the receipts and check the purchase date. Then you need to go to mfgs websites and learn about their warranty.
There are however other reasons why you should consider negociating a lower price per card, or buy just a few handpicked cards, or even ignore the deal. If those are mining cards, (for me) there's an huge difference in remaining value of card running at high voltage vs low. Where does those 550 Kh/s come from? If this comes from 6950 cards flashed to 6970, perhaps the core worked at 1.175V and rams were also overvolted, which can lead to artifacts and damage.
Then, there is miner software that doesn't pick 6950 cards, for example Memorycoin and Protoshares (?). With the incoming scrypt Asics, you need well supported, flexible cards that work with all other coins released now and in the future. When Maxcoin was (is?) the most profitable coin, I got 200 Mh/s on the 6950 and 230+ Mh/s on R9-270 and 7870 cards, with way lower voltage and power consumption.
$150 isn't such a good deal. 24/7, sustained 550 Kh/s is not possible in practice and it's actually a sign of abuse. The cards are less flexible than new Gigabyte R9-270 OC, or some Sapphires, or perhaps other brands. I have Gigabytes, I'm happy with them vs 6950, not vs 280x or 7950's, of course.
I have nothing else to say, as I don't know more or remember. Please don't send PMs for stuff like this, as I tend to ignore them.
