> people paying for things at stores is not because everyone has actually agreed on the price.

... I genuinely can't fathom what it's like to live in a developed country and yet have such little social trust.

You really imagine that when others are in line at a checkout, they have the intrusive thought "I could just bolt and not pay, but I see a security guard so I better stay in line"? You really have that thought yourself?

Of course people have agreed on the price. That's why you don't see anyone trying to negotiate the price, even though they would be perfectly within their rights to try. And it's why you do see people comparison-shop.