your wife would leave her full cart in the aisle and say goodbye. Also,some of those cashiers would fail miserably at their jobs if they had to negotiate, not to mention how long checkout would take.

but just because concentrating it feels weird, doesn't mean it isn't happening.

You pick up a costco Kirkland shirt. It's $45. You get to decide whether the quality and price fit what you want. You decide it doesn't, because you're a Sam's club shirt guy. You just negotiated with the price tag and left. When 2000 people did the same, costco lowers their price and then everyone can change their minds.

Since you went to extremes in your example, here's an extreme view of what were to happen if you couldn't negotiate at all. You show up at the store in the morning because lines are always ridiculously long, you are handed a standard pre-wrapped grocery package which includes bananas even though you hate bananas. You notice the flour allocation is decreased for the third month in a row. You ask if you can not pay for the bananas, the soldier laughs. Soldier scans your ID to make sure you didn't get your grocery package already that week, tells you to pay your $100 for groceries and give away what you don't want. Huge blackmarket exists where people can freely negotiate and where you can trade your bananas for cough medicine.