i've said this before, but i don't believe ebay prices are an accurate measure of value unless you averaged all the sales for a particular book over a long period of time. a couple exorbitant prices here or there could be flukes or they could be the beginning of a trend, but it's too little data to say. yes, ebay does tell you definitively how much someone is willing to pay, but it also tells you how much someone is willing to pay that week, and only from the group of people who happened to see the item that week, only from the group of people within that group who remembered to bid when the item was ending. i've had experiences before where i listed a book for $20 and it didn't sell, only to sell for $150 the next week when i relisted it. similarly, i have taken books to bookstores to trade in and been offered $8 - $10 in trade credit, only to sell the book on eBay for $100 or so. so many things are at play when an item sells on ebay - auction psychology, timing, how many similar items are listed or have been listed, etc. that it is reductive just to say that abebooks presents one range of price and ebay presents a much lower range.
i totally agree with mjp's point RE: information, but what i think we're seeing is that people not only know where to find better deals than what booksellers are showing them, but they are also going direct to the market with their demand, rather than involving booksellers. this is why someone at green apple books in san francisco - one of the two best stores in the bay area with hundreds of thousands of titles and seasoned buyers - tells me that there's no market for a book signed by dan clowes, jaime hernandez, and charles burns. because the market isn't going to them for that book, it's going to eBay and paying a lot for it. the book trade isn't operating on outmoded logic, they're just serving different customers, and those customers are paying book trade valuations. that doesn't mean that a bookseller is overcharging when they sell something for 2x what it sells for on ebay (well, not necessarily at least), because they're reacting to different customer demand. if i had sold my book to green apple, they would have put $25 on it, which doesn't mean the ebay bidder who bought it for $150 got ripped off either.
i don't know what my point is. maybe we're past "books are worth what people will pay for them" and more "book value is malleable depending on the venue, seller, and customer." i do like this discussion a lot, though, so thanks for permitting me my verbosity.