If you can abduct someone unobtrusively, the police will have no witnesses and will eventually have to resort to a 'usual suspects' list to see who the disappearance fits. If you did it, you're at risk of being caught only if you're actually on that list.
Attacking someone during a house viewing works for getting someone unobtrusively into a place you can harm her, but has two drawbacks. One, you probably leave a trail to where she went, the site will then be visited and traces of you found. A way to deal with this might be to change the viewing location at the last minute. You ring the office at 12.40 to say Actually could we see somewhere else? SJL immediately goes to the somewhere else and hasn't time to update the diary, which still says "37 Shorrolds O/S". The drawback to this is that if she did so, the keys to that different property would have gone missing.
Two, this approach doesn't make the victim disappear. Ideally you need a house or building you control, to which the victim is brought and then leaves in the boot of a car via the built-on garage. She goes in the door and is never seen again dead or alive. I think this is what the Moors Murderers did; they were only caught when they involved a third party in the murders.
