Bookride, a dryly funny and informative blog about the rare books trade, has a good entry about using the Web to check the value of a book.
The mistake most people make when valuing books on the web is to take their price form the highest or the mid range. None of the books listed have sold and if you had to buy one you would choose the cheapest in decent condition; only a mad man would choose to pay more than necessary. Take your price from the low end of books in comparable condition.
Who are these guys with absurdly high prices? Generally they have had unhappy childhoods, uncles who drank, boorish parents or have been educated at unpleasant and expensive schools.
I've noticed the same inflated thinking with comic books. When I worked at a comic book store as a teenager, people would come in with old comics that had torn or missing covers and be flabbergasted to learn that we would give them about 1/100th the amount they'd get for the same comic in near mint condition. A good rule of thumb for people who think they have a valuable collectible is, "if it's not in perfect condition, it's worthless to a collector."