The below tutorial will demonstrate how a seller can lock or unlock a contractor lockbox. I won't be getting any more complex than that below. I provide both written/pictorial & video including captions if desired.
[The below assumes that the lock is already correctly attached by an agent (i.e. to a door knob or even better, a wire, since I've seen where lockboxes directly on door knobs rubbed on the paint of the door)...]
1st 2 Steps for Locking or Unlocking Contractor Lockbox Instructions:
1st step for locking or unlocking: If combination #s aren't visible, open the small compartment to expose them by pressing down on the small lever
2nd step for locking or unlocking: Shift the combination numbers to be in line with the code, lining them up with the line on the unit.
Locking Additional Steps:
3rd step: Place the key in
4th Step: Close the lockbox
5th Step: Scramble the combination so that each digit is different.
6th Step: Close the small compartment to hide the combination digits by pressing up on the small lever
Unlocking Additional Steps
3rd Step: Press down on the small lever to the left, moving it visibly down
4th Step: While still pressing down on the lever, pull the face back/towards you (ideally with your other finger elsewhere, such as on the bottom lip)
5th Step: Remove the key if Present