1 A) if the next owner doesn't want root, restore a backup that was made before rooting. If you didn't make one yourself, you can do a forum or google search for a non-root backup (may have to word it a little differently, and no guarantee someone else's backup will work on your phone).
1 B) if they want root, proceed to step 2.
2) rooted or not, remove any SD card in the phone unless you're including it in the sale, then do a full "factory data reset". This will put the phone back as if it'd just come out of the box (except it will be rooted if you didn't do the backup step).
3) turn the phone off, then prepare it for shipping.
Good luck