Assuming it was a retail part number (p/n ends in -KR), you won't need to do anything to transfer the warranty.  Info/documents won't be needed.  
 
If the buyer needs warranty support or assistance, he/she can call into EVGA at any point.  The buyer can check the warranty status or setup an RMA any time by going to http://www.evga.com/Support/guestregister.asp.

Generally, they don't register it again.  They can register it if they want, or have tech support do it, but they can get full support via the link I posted above.  Whether you unregister it or not won't change anything.

You can unregister your card from your account if you wish, but as NordicJedi has pointed out, that will not change anything in our system, the card will still show as having been previously registered. However this will not change the warranty status of the card at all, as the warranty period will be determined by the ship date of the card from our warehouse.