If I see that the buy it now is a little higher than I think it is worth and they have a best offer, I usually come down about 10% (based on price point), and most of the time the seller counters at around 5%, just like buying a house.

Sometime if the seller must an absurd number on something I will give a low end, and leave a comment saying your price is too high, my price I believe is fair, etc, etc... and a few times they emailed me saying thanks for being polite, and accepted my offer, then again I've had some A-Holes respond back in a not so nice way.