So long as the seller has a realistic price on whatever item.
I used to offer half as a standard practice, one time I felt we were totally justified on a '72 Fury Grand Coupe. The sellers kid thought the car was gold, I talked to the old man after he left, informed about the market for such cars, and he accepted our offer. I actually felt kind of bad, but I think his kid was comparing the car to a Challenger R/T, or similar.
As I've gotten older, and have a better bank account, I'm more inclined to pay closer to asking prices, but still love the art of haggling.