Buyer fills in V5C then you both sign and date it, you give buyer green part and send rest off to DVLA. It is also advisable to issue buyer with a receipt which you both sign/date (make a copy to keep also)
SORN is cancelled when buyer takes ownership (although it will only be updated at DVLA once they have received and processed the V5C of course)
If buyer is driving the car after purchase then he/she needs to tax it or make arrangements with you to tax it.