There is a time and a place for haggling, but let's not pretend that it's not relatively common for someone to be beat or killed because the other party felt they got ripped off.
I'd argue that a higher percentage people have been hurt or killed over haggling disputes over direct sales disputes because the former has a higher emotional component and people feel slighted easier and more often.
I'd argue you have it exactly backwards.
Seller sells for less, that's his choice. Getting talked into it is his fault. C'est la vie.
Buyer gets shafted for a decent chunk of change - by not haggling, and not knowing what's a deal and what's to walk away from - now you have someone who got, as you said, ripped off, and might just be mad enough to do something about it.
I bought a pedal recently, rebought actually. Wanted to buy from a local shop, and had happened upon it checking out their inventory.
Guy says - they don't usually sell used items - that it's on consignment with an employee.
"Hey, how much you want for that pedal?" I hear the guy say 40 bucks. I expect a bit above that do they can make a small profit.
Guy goes and looks up a crackpipe seller on Reverb and says "$160, see?"
"Oh, well, let me think about it, just bought a Marshall, kinda tapped." I wasn't haggling with this nigger, I wasn't whipping my phone out and showing him 3 on Guitar Center for $49 and shipping, I bought my $2 of loose picks and never went back. Fuck you, retard. You get NOTHING now.
Months later I looked it back up, and sure as water is wet, there's 3 or more for $49. Knew when to walk away, and for all I know, that other example is still gathering dust in their case. I want to support mom n pop guitar stores, but they are more often than not just such greedy assholes, worse than pawn shops. A (((true haggler))) would have tried for the love of the game.
Imagine if I was just like "oh, sure! Shut up and take my money!" and then went home and saw the GC price. I'm sure somebody has had the opposite happen, a seller goes apeshit because the silver tongued devil "ripped him off", but the outlier doesn't debunk the trend.