Negotiation is a fact of life. If two people are starting new jobs at the same company filling the same role and one is a better negotiator they will probably start with a better salary, maybe more vacation time, maybe a starting bonus. Should this be illegal? Is it morally indefensible, or is it just the reality of competition in the real world and extends into many aspects of our lives even if we aren't always fully aware of it?
I don't think we're asking about the legalities or moral defensibility of a system. We're asking if there's a better one. In your unrelated example, would the company and potential employees be better served if the salary and benefits were fixed, given the job requirements? How would that change the interaction between the company and "hard" negotiators vs "soft" negotiators? Is it possible this type of negotiation is the cause for the pay gap between men and women, or other unfairness given to minorities? I think those are valid questions and dismissing them as "negotiation is how it is" doesn't permit the thought experiment.
You are right that some customers will benefit more from the Tesla one-price model as they don't have good negotiation skills... although, as I pointed out Tesla still has opportunities to take advantage of those customers like traditional dealership... most likely by under-valuing their trade-in.
Tesla undervalues all trade-ins. I think that's one area that most people aren't going to be taken in (at least not blindly), since real values are clearly available elsewhere. I think your argument is supportive of a negotiating-acumen socialist style format where the less-savvy consumers make up the bulk of the profits for the seller, and the savvy consumers get to walk away closer to "cost".
Personally, I feel like fixed pricing is a good model. The interaction between the salesperson and the consumer doesn't include as much positioning and self-interest, so in theory it fosters a more collaborative experience. There's also a level of fairness as far as knowing that others who bought your same spec on the same day paid exactly the same price. That said, I think the drawback is the monopoly aspect of the product. Tesla has a one-of-a-kind product so you can't really cross-shop it. And since they don't sell through retailers/dealers, it's not like you can cross shop them. It's a take it or leave it price, though I'd argue it's clearly market driven.
Buying negotiated items is generally listed as the least pleasant consumer experience. Cars rank at the top. Mattresses always make the list as well. Why? Because many people don't want to have to deal with negotiation when making decisions about an important product. They want a cooperative experience, devoid of suspicion. This is one of the reasons for the success behind
Danny Meyer's no-tip policies in NYC. It's not "is my server trying to sell me the great dish for the tip" or "is this customer going to tip me properly even though I'm not terribly happy today"? The servers at those restaurants have said that the customer/server relationship improved tremendously.
So as I said before, this is the most time I've spent thinking about it, but for now I think Tesla's model is a better model. I'm sure I'm missing some things, but that's my initial take. Sorry for all the thinking "out loud".
