How do I secure the keyboard and trackpad when I trade in my Mac

You don't need to include the keyboard, trackpad or mouse with a trade in. Apple does not want them. Keep them for spares/backups for the newer Mac.

OT is correct, you do not ship the keyboard and mouse. Also, make sure you have photographic documentation that the computer is in the shape you claimed it is in. In the event of shipping damage or a difference of opinion without any proof it's a you vs. them confrontation and guess who has deeper pockets?