It is not totally necessary to unregister a product if you sell it to someone as it can be registered twice.

But to unregister a product you would go through your EVGA profile under Member Home > My Products > View/Edit my Products > Click 'View Details' on the product in question and then click 'Unregister Product'.

Note: Per our warranty, only products purchased or shipped from EVGA after July 1st, 2011 would have a transferable 3 year warranty.
Products purchased prior to this date do not carry a transferable warranty.