Haggling is wrong.
To me, this is obvious, self-evident, and inherent. One plus one is two, water is wet, and the act of haggling over a price is morally and ethically wrong.
I feel weird, perpetually out of place, as though I've been teleported to a parallel dimension in which kidnapping and torture are commonplace and lauded, and I have to explain, as if to a small child, that these things are bad. It baffles me that I even have to bring it up, and baffles me still further that people don't agree with me.
Imagine that I have widgets to sell. I consider the price of raw materials, the cost of production, the current prices of my widget competitors, the amount of profit I need to make to stay in business, and I determine that the optimum price at which I can sell widgets is five dollars. I set up my pile of widgets and I post a sign next to them that says "Widgets: $5 each".
Now, imagine you come up to me and ask if you can have a widget for four dollars, as if the revolutionary concept of selling widgets for four dollars simply had not yet occurred to me. If you're not offering me anything further in exchange for that discount - a large volume purchase, for example, or a guarantee of future purchases at regular price - this is not functionally different from just asking me for a dollar. Asking for money and not offering anything in exchange is begging.
Adults with jobs should not beg*.
Haggling is unpleasant. No one enjoys price negotiations. They are not fun conversations. Discussing money with strangers is stressful, irritating, and time-consuming. Engaging someone in an unpleasant activity, particularly if they did not explicitly ask to be involved in that activity, is not a nice thing to do.
Haggling is inefficient. Imagine a supermarket in which everyone is allowed and encouraged to haggle with the cashier - you wouldn't want to go to that supermarket, would you? Sure, you might possibly save ten or twenty dollars on your food bill, but your time is valuable (isn't it?) and that sort of supermarket would, necessarily, have you waiting in long lines before you could talk to the cashier, and then you'd be stuck there arguing with him about the value of a can of tuna for a long time after that, all for a possible discount that you may not even get.
After all, regardless of how shrewd a negotiator you may think you are, perhaps that cashier is having a bad day, or has been instructed by his superior that his job depends on bringing up his revenues, or maybe he just doesn't like your face. Perhaps you offer to pay 80% of the listed price of your tuna, he counters with 120% and doesn't budge. You'd either have to pay it, or you'd have to go put your tuna back, walk out of the market without the fish you needed, and you'd have wasted at least an hour.
Haggling makes prices inconsistent. Let us suppose that I have a strict budget, and that budget for this week has a column that says "Food: $100". Now, suppose I shop at the supermarket mentioned in the previous example, and I didn't do particularly well at haggling this time around - to buy the groceries I want, it'll now cost me $130. I have to choose between spending this week feeding my toddler ramen, or taking $30 out of the column of my budget that says "Rent" or "Utilities" or "Savings". Thirty dollars may not be much to most people, but for those who are struggling with debt and living paycheck to paycheck, it can mean the difference between hot dogs and cat food. In a world in which one has to haggle for basic necessities, precise planning and budgeting become impossible.
Haggling is unfair. The world is already tough enough on people with disabilities, people in ethnic or cultural minorities, people who - in D&D speak - are suffering a circumstantial penalty to Diplomacy checks. Forcing them to pay more for basic goods and services simply because they cannot vault over a merchant's conscious or unconscious prejudices against them is cruel and unusual. I am young, white, male, visibly straight, and eloquent in the English language - and all of these facts are due entirely to luck and none of them should net me a discount on baked goods.
Haggling encourages dishonesty, or at least insincerity. People who are engaged in the practice will usually feign friendliness with their opponent (usually, a complete stranger whom they have no intention of seeing again). They'll play up the dire financial straits they're in (often, a litany of unverifiable complaints about unrelated misfortunes). They'll exaggerate how angry they are about the prices of things (Yes, I know this isn't the same price it was five years ago, prices always change over time). They'll mention that they'd seen the same item for much cheaper elsewhere (why didn't you buy it there, then?). They'll promise that they'll tell all their friends about how wonderful the service they received was (so I should sell you my widget at a loss, so I can be innundated with all your friends who will also insist that I sell them widgets at the same loss?)
People who are haggling with you are not your friends. Friends don't have to haggle. Friends don't subject each other to haggling, and they especially don't subject each other to the guilt and awkwardness of implying that a discount would be in direct proportion to their friendship.
Everyone who writes a utopia necessarily includes their biases and their opinions, showing a fictional world in which everyone conforms to their ideals and the world is better for it. In this way, utopian fiction is perhaps the most revealing of genres. Lily is not me, and Florenovia is not my utopia, but on this matter we agree - if I was to create a utopian society, haggling of any sort would not be a part of it.
*(unless it's for sex)