I haven't been burned in my dealings but then I don't bother calling if the machine is priced above what I'm willing to pay. Every situation is different. If I run across one at a swap meet, yard sale, something of that nature and have cash in hand, I'll negotiate. I tend to be honest with them. I tell the owners I have other machines and how the market really is. Their eyes glaze over after a few minutes and they lower the price to get me out of there.
I've found muttering to myself helps too.
Craig's List, I don't call unless I'm ok with the price and I don't haggle any if at all.
I figure the person will be less likely to sell the machine out from under me if they're getting full asking price. Too many of the "make offer" type people are hoping for a bidding war. I usually won't bother. The one's citing Ebay as their price guide really don't get my attention either, they tend to be over priced.
Rodney