They only need the last 4 digits though, which is fine:
If you wanted to sell them at face value or below, or transfer them to a friend that's fine. You would need to write the new owners a letter of permission allowing them to check in on your behalf when they arrive. They would need the physical tickets, booking reference number, personal ID to prove they are the person in the letter and also a photocopy of both sides of the card used to buy the chalet to show upon check-in. You can delete all but the last four digits of the main number on the card for security on the photocopy.
They only need the last 4 digits though, which is fine: