I'll haggle over the price of a car and a house, but that's about it. I've only bought a couple of things on eBay, and I never bid, I just paid the "Buy it now" price if I thought it was worth it.
For what it's worth, we don't buy a lot of stuff, and what we buy usually comes from a store or online retailer, not an individual. So on the occasions when we do buy from a private entity, we just pay what they're asking or we don't buy it.