It depends on what you're selling. I feel it's fair to haggle and deal only on a big purchase like, say, a car. Something that costs upwards of a couple of paychecks, at least. I tend to pay cash for things like that as much as I can, and since that means there's not gonna be a delinquent payment and they get their money in full on the day of the sale, they usually cut me some slack and give me a better deal.
Plus, the sticker price is the sucker's price on a car or what have you.
But in a fixed-price, retail setting there is no haggling and I understand that if that's what they charge, that what they charge. No matter how much I bitch, Wal-Mart isn't gonna take 5 cents a can off my Hormel Chili.