I think there are different trading models, and it's often difficult to distinguish which applies in particular circumstances. I remember buying a camera and being surprised when a couple of free films were thrown in with it. Later I heard that it was standard practice, not just that dealer. And with cars, there's always a lot of fat bilt into the price to allow for haggling. If you're in a situation where haggling is the norm, then failing to haggle means you're losing out.
But it's the seller who determines which model applies. And this can lead to incorrect expectations from a customer. As a customer I like to see where my money's going - and to check that the seller, often unknown, is charging a fair price, not an inflated one as often happens.
Here I'd say that the haggle/sweetner selling model would be a reasonable expectation for a buyer, especially as a lot of new saxes are sold with cheap extras thrown in - strap, mouthpiece, reeds, cleaning kit... It's not a fixed price supermarket deal (and even they often have specials, two for the price of one and so on). But as I said above, on a commission sale, things may be different. And here it clearly was. Pity the salesman didn't handle it better - it led to confrontation and dissatisfaction/ill feelings. And if the sax hadn't been such a good match, the sale would have been lost. Knowing in advance that I'd be dealing with a salesman like that would certainly put me off going in in the first place.
So although I agree with you, unless you know the seller, it can be difficult to know what to expect.