In the case of the car, it's definitely not a one-off purchase. People will go back to the dealer to get the vehicle serviced, and from what I understand this can (mainly for new cars) have a greater total lifetime value than the cost of the vehicle itself.

Even if this weren't the case, if you're ripped off by a used car salesman you will tell every single person you know that they're a snake.

Even if you don't do that, there are arbitration courts you can take them to to both waste their time and force a refund (VCAT where I am, I'm sure there's something similar overseas).

I really don't think it's in a used car salesman's best interests to lie and swindle.

You're mixing situations. I was imagining a NEW car sale, not used cars. In that case the product itself isn't in control of the salesperson, so there's no room for swindles there - but you have an adversarial relationship in terms of price, features / upsells, etc. That said, you're hardly going to blame the dealership if you get talked or dark-patterned into an upgrade you don't need or even regret. "Caveat emptor" and all that. We're so used to adversarial situations like that we blame ourselves more often than not.

A collaborative approach is a lot more like when you ask you're knowledgeable friend "what kind of bike should I buy?" and they ask you about your needs, goals, and budget then guide you towards specific options. They're helping you make a purchase, without pressuring you into something that's not useful to you.

>adversarial relationship in terms of price, features / upsells, etc.

I get what you mean now. As someone who "does sales" a fair amount in my work (owner of a second-hand video games business) the adversarial stuff is actually really frustrating.

I always try to give the customers the same advice I'd give a friend or family, even "downselling" them from a $100 purchase to a $25 repair if that would be more suitable, yet still so many people are adversarial and distrustful.

From my experience, this distrust hurts (non-expert) consumers even more than it hurts the businesses. Ideally, you'd have an honest salesman pointing out the pros and cons of all the different products so that the consumer can make an informed choice. Maybe that product that costs $50 will last for 20 years of continuous use, whereas the $30 version will break after 18 months.

Instead, because salesmen can't be trusted, they rely on their own instincts and buy products whose prices are completely uncorrelated with quality.