The first thing to remember is that most books are of low value or no value. Some books are worth less than nothing. A quick look on ABE (or in the case of newer books, Amazon) will ascertain whether the book is common or not. In the case of a book of negligible value the screen will fill with copies with prices starting at £5 or less, sometimes at £0.01. Prices less than this are not permitted. Do not (at first) put in too much information -author's surname and part of the title will do (e.g. Steinbeck /Wrath) with a few boxes ticked such as 'first edition' 'Dust Jacket' etc., Too much information entered can lead to the impression the book is rarer than it is --this is a ploy, by the way, sometimes used by canny sellers to demonstrate a book is more valuabe than it really is. Beware.
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 anybody who had to buy one would choose the cheapest in decent condition; only a mad person would choose to pay more than necessary. Take your price from the low end of books in comparable condition. Considerations of postage and proximity may then be taken into account and you might pay a little more to a reputable, proven dealer. If you were selling the book a dealer might give you between a third and a half of the low price, or if you were to sell the book on Ebay you might achieve half or possibly a tenth and in some cases nothing. Once in a blue moon you will get way more than this, but you will almost never achieve the highest price--nor will the seller even if he waits 150 years.
What about if the book is not on the net? You may have a prize or something so obscure that punters for it are non existent. You can leave the book as a want at ABE and be informed when one shows up but it may take years. In the case of an obviously rare and desirable book you can consult auction records in a library or consult a venerable dealer (preferably from ILAB, ABA, ABAA, PBFA or some recognised book association.) You are not obliged to sell to them and they may charge for an appraisal (usually waived if they buy the book.)
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. Until the internet came the truly greedy dealer could not make a living as no one would buy from them. Now it is the Wild West out there; although charging an absurd price seems like poor business as your cash flow will be a mere trickle. I am not talking about renowned dealers with fabulous stocks--their prices, although high, are seldom insane and they often have the best stuff. Any serious collector will occasionally have to buy from them and they will sometimes offer terms. It is worth keeping a shit-list of malodorous overchargers as their prices often distort the first few prices in the list--I won't name them but avoid the likes of Len's of Bournemouth, Wainwrights of Peaktown, Books of Venture, Rapturous Editions of America, Books of William Why and various bookbarns, sheds and shacks...not forgetting Attic Books of NH - in a vigorously contended field the world's most overpriced seller.
Good luck. Apart from ABE and Amazon I recommend megasearchers such as the excellent viaLibri which can also take you to world libraries for further research. Also Bookfinder and Addall are very useful. For sobering or unpredictable prices check if copies are currently being sold on Ebay. Lastly Google can sometimes uncover copies for sale on independent sites run by oddballs who have not joined the bookmalls...