It's all Breaking and Entering.

That's how the owner views it, if you use the combination lockbox, and enter their home without a preapproved appointment.

Calling to make the appointment, and going to the house without waiting for approval is a big no-no.

And if the seller wanted to make a big deal about it... well, bigger than calling their agent and reaming him/her a new exit-hole... they could call the local constabulary, and file charges.

That's right... it's breaking and entering.... B&E... well, okay, maybe it's not "B", if you already had the combination from a prior showing (or an electronic lockbox key)... you're not really "Breaking" anything (other, I suppose, than the seller's trust)... but you are entering against the seller's wishes, and you could find yourself (and your buyers) enjoying the hospitality of stale cookies and even staler coffee, down at the local police station.

You could also find yourself the subject of a grievance filed with your manging broker, your local board, or even the state governing board, costing you thousands of dollars, and potentially your license.

Don't do it.