Personally, what I really like is big manufacturer rebates. That way we both win!
I got almost 40% off msrp on my car, about 20% off on my atv, but I had them throw in a winch free. I got a pretty good deal on my tractor too, so I didn't haggle much on the extras like the tire loading and the coolant heater. He's got to keep the lights on too.
Auctions for complicated devices scare me as I know that's where I'd send my junk, otherwise I sell stuff privately.
I don't really like negotiating for a long time so I usually just state my price and if they accept or get very close, we have a deal.