I would NEVER list an item with a firm price. Why limit your potential market! Everybody likes to haggle (remember the marketplace scene from "Life of Brian") and feel like they've saved some money...it's a psychological thing! Sellers should allow some "wiggle room" when pricing. Sometimes, splitting or covering the shipping charges is enough to close a deal.
Remember too, that used pricing guides are just that, guides! Firm pricing should be used for rare, collectable, or unusual gear...don't limit interest in your item.
I can usually tell if the potential buyer is a tire kicker doing research or is really interested in the item. Feedback, frequency of discussion posts, and length of Agon membership are all good indicators of a truly interested buyer. Usually, tire kickers will not be very familiar with the item for sale, ask some kind of goofy question, or make an irrelevant comment about their financial hardship. Hey, high-end audio is a luxury hobby. I don't care if your girlfriend just left you, your refrigerator just went out, your dog just pissed on and ruined your carpet, etc. I don't mean to sound callous, but if it's a problem with health, shelter, or hunger, THEN I WILL CARE!