Have you collected a movement book from your local Department office? If not, you need to do that. Inside you'll see the sheets are in triplicate, you fill in the seller bit (most of the form), and tear off the white and yellow bits and give them to the buyer, leaving the pink bit in your book. The buyer then completes the rest of the form when they get home and sends (or scans and emails) the white bit to SAMU, and keeps the yellow one.
You don't have to notify anyone when selling, just update your flock record and keep with pink slip for 3 years as evidence to back this up if you have an inspection.