Erase Hard Drive on Mac Air for trade in

See instructions in What to do before you sell, give away, or trade in your Mac - Apple Support

press option on startup

release when you see the apple logo

i would search this topic in the apple website just to make sure.

there is a disk utility option were you can erase the drive.

what apple never includes in their instructions is to restart the MacBook and try to log into iCloud.

if you could, re-install the version that came with your laptop which should be el cap.

hope this helps