Imho, gone are the days when one could just simply call up a realtor and ask to view a home. Most don't just hang out waiting for the phone to ring from unvetted strangers wanting to view 7 million dollar homes, unless they know that the effort can potentially result in a sale. Most realtors do nowadays require a contract before they will even bother (buyer representation agreemnt) showing a home. There is also financial vetting. Most people nowadays have preapproval for the mortgage they can qualify for, which, along with down payment information, is shared with the realtor who is going to be spending his/her time showing the home.
A home of that value would preclude many potential homebuyers from even looking, no matter how curious they might be or how much they might want to see a home, simply because it is out of their reach and a waste of the realtor's time.
Nothing is to say that false info can't be given, and it doesn't likely go as far as requiring ID to get a buyer agreement, but realtors also have their gut instinct for detecting time-wasters. Sometimes homeowners will require certain information on those who view their homes, and there are expectations that only those persons who actually have an interest and a means to potentially purchase the homes will be viewing them.
I am not speaking about 'open houses', since I don't have any information to say that an open house had been held, and personally, I can't imagine that this home and the Shermans would have offered one. However, even then, realtors are expected to obtain information on who viewed the home. There are some little articles regarding open houses here:
Open house - RECO Website
Obviously the Sherman couple were still living in the home at the time when the home was for sale, although they *were* soon to be headed out of town for a few weeks. Realtors are expected to ensure potential buyers are not given opportunity to steal anything, ie they don't just stand at the door and send them off to wherever they want to go on their own, they are instead accompanied throughout the house.
It is possible that one of the potential buyers who was shown the home could have been viewing it for the express purpose of 'casing the joint', to see exactly where any cameras were, where exterior access points and certain rooms were located, etc. Perhaps not even for themselves, but to report to others. Hopefully TPS is having a close look at anyone who accessed the home in the time before the murders. jmo.