Really depends on the object. If it's a collectible item with a value that's open to interpretation, I sometimes do, especially if I'm considering buying multiple things. (For example, CCG cards priced at $20, I might offer $70 for a playset of 4.) Those things don't have firm market value (or that value fluctuates frequently) and there's usually an easy way to look up a price range quickly to get a sense for what's a fair or reasonable offer.

If it's something someone made and is selling, it feels rude to me to haggle. The item has no real market value because it's something they made; the price is what they're willing to sell it for. I'll either buy it for that price, or not buy it at all. I guess the exception would be if they've got a sign inviting haggling, which I've seen at convention spaces on rare occasion.