And ebay is not always the best source. I have seen alot of people have friends run the price up on auctions. You can usually tell when you look at the bidding history and see new users or zero feedback bidders involved. Most times those bikes pop up repeatedly. Research is the best answer. I know nothing about airheads, so I asked just this weekend for input on a value. With Triumphs and BSAs I watch and look constantly, so I have more of a grasp what they should bring.
Time of year and location come into factor as well. If a buyer on ebay is local they will most likely pay $500 more for a bike they can drive and pick up. Shipping is a major price factor. I have seen several bikes I would love to have at good prices, but when you figure 5-800 to ship it, its not such a good deal.