The sales guys are still often idiots. But if you know the cost of the car, don't need financing, and either plan to do a private sale of your current car or it's a donate for $2K vs. trade-in situation, it isn't really all that stressful.
What's stressful is when they start playing games with all the levers. If you're fortunate enough to not be in the position to maybe have to play those games, you basically hold the cards.
Being a former certified car sales guy who changed to IT out of passion, I am currently in the process of buying two cars for business and family. I am horrified by the current state of car sales. Cars are presented dirty, salespeople show absolutely no interest in even talking to us while staying around their dealership for a long time looking at specific cars.
Best was this Monday, when I called one dealer and told: I want to buy a (specific) new car. Now. No haggling, I just sign. Had to call them 3 more times, every time I got a new excuse why no salesman is available to me. "Please wait another hour, then he is out of the meeting"... so I just called another more expensive dealer after 8 hours, told them the same thing. The salesman sent me the contract in a second. But as I mailed the signed contract back after business hours, I did not hear anything from it the next day. Called the dealership after waiting half of the next day to hear "Salesman out of office today". I said: no problem, I just resend the contract to another person. "You can do this, but the salesguy needs to get permission from the boss to sell you the car". At the end of the day, they somehow managed to send me the invoice for the car.... but everything took many phonecalls.
Only this can create problems of its own. Some dealers will refuse to work with you if you are paying with actual money instead of a loan. My parents went through this. They sold some farmland and decided to buy a new car with the profits. A few dealers simply said no. The car they wound up with mostly depended on where would actually sell them the car.
You could also be missing out on incentives from the manufacturer for not using their financing. It's definitely a good idea to have money or financing already lined up, but some manufacturers will give thousands of dollars in incentives to use their financing.
There are probably cases where the current incentives are rolled up in the financing, e.g. 0% interest. But there are also often other fees and, frankly, the available (effectively) risk-free investment options these days have such low rates of return as to be almost indistinguishable from 0%.
They're not giving you deals on the financing because they like you.
If I were buying a car my very strong default is to just pay cash and be done with it.
Those incentives come with costs. In their case, it was better to know they didn't have to worry about making a loan payment every month for years when their overall financial future was a little iffy at the time - prudent when the main income earner's health was failing. It wound up being the correct decision, as my father died a couple years later.
My last vehicle I had cash to pay for it. Manufacturer offered a $2k incentive to finance through their own financing arm. I asked, and there was no early payoff penalty. So I financed the car, and sent a check a week later, paying it off completely well before the first payment was due.
They bank on screwing people over with financing. I didn't tell them how I'd be paying until we were signing paperwork already. He wasn't happy to hear a I will be writing a check. Then I refused to sign the arbitration agreement and he really got mad, but oh well I got the car anyway. However it wasn't a pleasant experience overall.
Some simply aren't willing to do the paperwork one must do with transfers over 10k. More importantly, they make more money off of the loans than they do with cash. This is true even if they must repo the car later on - they can often sue you for at least part of the total cost, minus the amount they sell the car for (that last bit might vary by state, though).
Also, when people say they're paying "cash" for a car they don't necessarily mean they're walking in with a stack of hundred-dollar bills. "Cash" often means cashier's check, etc. - the idea is that you're not borrowing money to buy the car.
Ugh. I was test driving a car I knew almost nothing about recently. I'm in the market for a fun weekend driver as a second car, and this one caught my eye on the dealer lot. I was looking at the car when a salesman approached me (I did not approach him, and I did not look at any other cars). I asked him to let me take it for a drive, and while we were driving I was asking him questions about it. "I don't know" was his only response.
I'm sitting here in the drivers seat of a two-seater sports car doing 110mph down the highway and this guy is completely silent. "Does it have a turbo?" I don't know. "How big is the engine?" I don't know. "How many gears does it have?" I don't know. "How fast can I be going while putting the roof down?" Actually on this one his response was "let's give it a shot" and started lowering the roof at highway speeds. Luckily it apparently can handle that.
Turns out the guy just started working there about two months prior and had never even heard of the car we were driving, he'd only sold Jeeps and pickup trucks and sedans up to that point. He just wanted the commission from selling a car he knew nothing about.
What's stressful is when they start playing games with all the levers. If you're fortunate enough to not be in the position to maybe have to play those games, you basically hold the cards.