"Here, madam, is a handy-dandy, super-duper, Grade A Deluxe Combination Slicer/Dicer/Chopper/Masher/Mixer/Juicer/Blender for only $49.99, and don't you agree, ma'am, that with just this ONE item in your kitchen you could do what you'd otherwise need six or seven OTHER space-hogging appliances to do?"
First, I trust you see the tautology. This device, by its very name, does seven different tasks which, one supposes, could also be done by seven separate devices were you to make it a serious point to track down seven devices that each accomplish only one task. So the salesman is in essence saying, "This one device does the tasks of a hypothetical seven separate devices; do you agree with me that that is what I've just SAID?"
And the poor "pigeon", overwhelmed by both the machine-gun delivery and the apparently unassailable logic, stammers, "Um, er, well, y-yes, I suppose so...."
So now she's been cognitively coerced into taking the salesman's side. She could hardly help but acknowledge that the man just said what he just said, but the question was so formulated as to make it seem she was agreeing that she NEEDED this device on her kitchen countertop. There's little hope of her escape now.
Once you've taken someone's side, willingly or not, the fear of looking either fickle or unintelligent, or even like a traitor, takes on its own coercive force. Unless you are exceptionally self-aware, self-possessed, rational and healthily cynical, it is almost impossible at this point to back out.
There is another fear at work here, too. Or perhaps not so much a fear as a specious moral compunction. The salesman fired the tautology at the poor lady, asking her in essence whether or not he just said WHAT HE JUST SAID, so how could she HONESTLY say "No"?
A person's conscience kicks into gear. A certain intellectual self-respect and deep taboo against lying take over, so that you get this visceral resistance to telling an untruth: "How can I say to this man that I DON'T agree with what he said when what he said is obviously true? I'll look like a liar, or an idiot!"
Which of course is precisely what the salesman is banking on. He knows full well that what he just said is rubbish, and he doesn't respect you in the least for answering "truthfully"; all he cares about is that you follow the script right up to the part where you fork over the cash.
Which is why, in situations like this...and perhaps the more perspicacious of you out there will catch that I'm not just talking about pitches for slicer-dicer-blenders...we must develop a kind of thick skin, an imperviousness, indeed a calloused disdain (callouses ain't always bad; guitarists depend on them), with regard to such manipulative playing on our consciences.
The thing to say to the salesman...or anybody else...who springs this sort of "Don't you agree...?" line on you is, very simply, "No." And if the con artist splutters in indignation at your blatant denial of the "facts", well...so what?
As Miss Jean Brodie would say, "It doesn't signify."
It doesn't "signify" whether the odd factoid he threw into the mix was, on some absurd, puerile level, "true". The proper response is still, "No, I don't agree. I don't even care. I categorically reject this whole pitch."
Even when it feels like lying (which, remember, is how it was set up to make you feel), the right thing is still to say "No."
Later, upon calm, collected reflection, you'll realize it wasn't lying at all.
To say "No" to manipulation, whatever shape it takes, is ipso facto to speak truth.
