I'm to the point where I clearly state in all of my ads, "Price is FIRM, Cash Only, No Trades". While that doesn't stop everybody from trying to haggle, I stick to my listed price. One thing that always seals the deal when people come over "to see" my snowblowers is showing them my 35 point checklist of service items. I don't BS any prospective buyers and won't tolerate BS from them either.
Having said that, 99% of my buyers are the nicest, most honest people you could ever meet. It's the 1% of lowballers, scammers and fast talkers that ruin it for everyone.