I really don't mind an honest offer, lowball or not. The ultimate choice to take it is mine after all, and I know I would jump without hesitation on the chance to get something for half of the posted price. I do make lower offers, but try to keep it realistic. If it's worth about half of what they're asking, I'll give them my number and what I'm willing to pay so they can call if it doesn't sell.

I do dislike when people seem to feel that you owe them a discount because it's not exactly the right size, it's a gift for their kid, or because they drove a ways to look at it.
Other 'offers' I've had. "I really only want the rim/derailer etc, will you sell just that and how much do you want for it?" (Hmmmm, bike's listed at $150, so for just the rear derailer, how does $125 sound?)

Ken.