I once bought some video arcade machines from a guy and when I was picking them up, I saw a 2004 stern sitting there. I told him I'd give him $800 for it and he said he wanted to keep it. I called him a month or so later to see what he had and he said he'd now take that $650 offer I gave him a month ago.
Needless to say, I got a great game at a great price.
I never feel offended by a lowball offer. Sometimes that's all the money people can afford. Sometimes they are just too cheap to own what I am selling. I want to close deals.
And when on the buying side, I often offer less than half what they are asking. I just lay it out that I'm buying for resale and I list off some parts costs and the few hours of work I'll have to do in order to sell it. Then I tell them that I'll also list it on CL for $x, but I'll probably end up taking about 3/4 what I'm asking if someone has the cash in hand.
People are reasonable when you are reasonable. If someone is afraid that I'm going to make a few hundred off "their" machine, I don't need to be giving them the money I make by selling someone else's machine at a profit.
When someone lowballs me, I just say what the lowest I'll go and then tell them that they don't get warranty, they don't get consultation and if they want me to fix it, they pay shop rate. There's really nothing to get huffy and offended about. A lot of times I'll just say they should probably just look for a different machine.... and then they ask me where to find one.
Everyone wants a good deal.. many only settle on a great deal. Who am I to judge that which I do?